diff --git a/beacon-chain/rpc/eth/beacon/handlers.go b/beacon-chain/rpc/eth/beacon/handlers.go
index fc12a5e6cd..349ada614c 100644
--- a/beacon-chain/rpc/eth/beacon/handlers.go
+++ b/beacon-chain/rpc/eth/beacon/handlers.go
@@ -1220,7 +1220,7 @@ func (s *Server) GetStateFork(w http.ResponseWriter, r *http.Request) {
 	fork := st.Fork()
 	isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		httputil.HandleError(w, "Could not check optimistic status"+err.Error(), http.StatusInternalServerError)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 	blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
@@ -1331,7 +1331,7 @@ func (s *Server) GetCommittees(w http.ResponseWriter, r *http.Request) {
 
 	isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 
@@ -1512,7 +1512,7 @@ func (s *Server) GetFinalityCheckpoints(w http.ResponseWriter, r *http.Request)
 	}
 	isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 	blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
@@ -1686,7 +1686,7 @@ func (s *Server) GetPendingConsolidations(w http.ResponseWriter, r *http.Request
 	} else {
 		isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 		if err != nil {
-			httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+			helpers.HandleIsOptimisticError(w, err)
 			return
 		}
 		blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
@@ -1742,7 +1742,7 @@ func (s *Server) GetPendingDeposits(w http.ResponseWriter, r *http.Request) {
 	} else {
 		isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 		if err != nil {
-			httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+			helpers.HandleIsOptimisticError(w, err)
 			return
 		}
 		blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
@@ -1798,7 +1798,7 @@ func (s *Server) GetPendingPartialWithdrawals(w http.ResponseWriter, r *http.Req
 	} else {
 		isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 		if err != nil {
-			httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+			helpers.HandleIsOptimisticError(w, err)
 			return
 		}
 		blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
@@ -1851,7 +1851,7 @@ func (s *Server) GetProposerLookahead(w http.ResponseWriter, r *http.Request) {
 	} else {
 		isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 		if err != nil {
-			httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+			helpers.HandleIsOptimisticError(w, err)
 			return
 		}
 		blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
diff --git a/beacon-chain/rpc/eth/beacon/handlers_state.go b/beacon-chain/rpc/eth/beacon/handlers_state.go
index 93238f1139..fec9fea1d8 100644
--- a/beacon-chain/rpc/eth/beacon/handlers_state.go
+++ b/beacon-chain/rpc/eth/beacon/handlers_state.go
@@ -56,7 +56,7 @@ func (s *Server) GetStateRoot(w http.ResponseWriter, r *http.Request) {
 	}
 	isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 	blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
@@ -125,7 +125,7 @@ func (s *Server) GetRandao(w http.ResponseWriter, r *http.Request) {
 
 	isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 
@@ -227,7 +227,7 @@ func (s *Server) GetSyncCommittees(w http.ResponseWriter, r *http.Request) {
 
 	isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 
diff --git a/beacon-chain/rpc/eth/beacon/handlers_validator.go b/beacon-chain/rpc/eth/beacon/handlers_validator.go
index 6ca3ecc973..25a40b4892 100644
--- a/beacon-chain/rpc/eth/beacon/handlers_validator.go
+++ b/beacon-chain/rpc/eth/beacon/handlers_validator.go
@@ -44,7 +44,7 @@ func (s *Server) GetValidators(w http.ResponseWriter, r *http.Request) {
 
 	isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 	blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
@@ -222,7 +222,7 @@ func (s *Server) GetValidator(w http.ResponseWriter, r *http.Request) {
 
 	isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 	blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
@@ -258,7 +258,7 @@ func (s *Server) GetValidatorBalances(w http.ResponseWriter, r *http.Request) {
 
 	isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 	blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
@@ -419,7 +419,7 @@ func (s *Server) getValidatorIdentitiesJSON(
 ) {
 	isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateId), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 	blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
diff --git a/beacon-chain/rpc/eth/debug/handlers.go b/beacon-chain/rpc/eth/debug/handlers.go
index 639cc5af94..7a28a2da98 100644
--- a/beacon-chain/rpc/eth/debug/handlers.go
+++ b/beacon-chain/rpc/eth/debug/handlers.go
@@ -46,7 +46,7 @@ func (s *Server) getBeaconStateV2(ctx context.Context, w http.ResponseWriter, id
 
 	isOptimistic, err := helpers.IsOptimistic(ctx, id, s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		httputil.HandleError(w, "Could not check if state is optimistic: "+err.Error(), http.StatusInternalServerError)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 	blockRoot, err := st.LatestBlockHeader().HashTreeRoot()
diff --git a/beacon-chain/rpc/eth/helpers/BUILD.bazel b/beacon-chain/rpc/eth/helpers/BUILD.bazel
index ea727b703e..763472b6c4 100644
--- a/beacon-chain/rpc/eth/helpers/BUILD.bazel
+++ b/beacon-chain/rpc/eth/helpers/BUILD.bazel
@@ -12,6 +12,7 @@ go_library(
     deps = [
         "//beacon-chain/blockchain:go_default_library",
         "//beacon-chain/db:go_default_library",
+        "//beacon-chain/rpc/eth/shared:go_default_library",
         "//beacon-chain/rpc/lookup:go_default_library",
         "//beacon-chain/state:go_default_library",
         "//beacon-chain/state/stategen:go_default_library",
@@ -21,6 +22,7 @@ go_library(
         "//consensus-types/primitives:go_default_library",
         "//consensus-types/validator:go_default_library",
         "//encoding/bytesutil:go_default_library",
+        "//network/httputil:go_default_library",
         "//time/slots:go_default_library",
         "@com_github_ethereum_go_ethereum//common/hexutil:go_default_library",
         "@com_github_pkg_errors//:go_default_library",
@@ -40,6 +42,7 @@ go_test(
         "//beacon-chain/blockchain/testing:go_default_library",
         "//beacon-chain/db/testing:go_default_library",
         "//beacon-chain/forkchoice/doubly-linked-tree:go_default_library",
+        "//beacon-chain/rpc/lookup:go_default_library",
         "//beacon-chain/rpc/testutil:go_default_library",
         "//beacon-chain/state:go_default_library",
         "//beacon-chain/state/state-native:go_default_library",
@@ -57,5 +60,6 @@ go_test(
         "//testing/require:go_default_library",
         "//testing/util:go_default_library",
         "@com_github_ethereum_go_ethereum//common/hexutil:go_default_library",
+        "@com_github_pkg_errors//:go_default_library",
     ],
 )
diff --git a/beacon-chain/rpc/eth/helpers/error_handling.go b/beacon-chain/rpc/eth/helpers/error_handling.go
index 84222ac6d8..cd6b9ff46f 100644
--- a/beacon-chain/rpc/eth/helpers/error_handling.go
+++ b/beacon-chain/rpc/eth/helpers/error_handling.go
@@ -2,11 +2,14 @@ package helpers
 
 import (
 	"errors"
+	"net/http"
 
+	"github.com/OffchainLabs/prysm/v6/beacon-chain/rpc/eth/shared"
 	"github.com/OffchainLabs/prysm/v6/beacon-chain/rpc/lookup"
 	"github.com/OffchainLabs/prysm/v6/beacon-chain/state/stategen"
 	"github.com/OffchainLabs/prysm/v6/consensus-types/blocks"
 	"github.com/OffchainLabs/prysm/v6/consensus-types/interfaces"
+	"github.com/OffchainLabs/prysm/v6/network/httputil"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
 )
@@ -28,6 +31,22 @@ func PrepareStateFetchGRPCError(err error) error {
 	return status.Errorf(codes.Internal, "Invalid state ID: %v", err)
 }
 
+// HandleIsOptimisticError handles errors from IsOptimistic function calls and writes appropriate HTTP responses.
+func HandleIsOptimisticError(w http.ResponseWriter, err error) {
+	var fetchErr *lookup.FetchStateError
+	if errors.As(err, &fetchErr) {
+		shared.WriteStateFetchError(w, err)
+		return
+	}
+
+	var blockRootsNotFoundErr *lookup.BlockRootsNotFoundError
+	if errors.As(err, &blockRootsNotFoundErr) {
+		httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusNotFound)
+		return
+	}
+	httputil.HandleError(w, "Could not check optimistic status: "+err.Error(), http.StatusInternalServerError)
+}
+
 // IndexedVerificationFailure represents a collection of verification failures.
 type IndexedVerificationFailure struct {
 	Failures []*SingleIndexedVerificationFailure `json:"failures"`
diff --git a/beacon-chain/rpc/eth/helpers/sync.go b/beacon-chain/rpc/eth/helpers/sync.go
index 864d11dfe1..c54bd89c7a 100644
--- a/beacon-chain/rpc/eth/helpers/sync.go
+++ b/beacon-chain/rpc/eth/helpers/sync.go
@@ -56,7 +56,8 @@ func IsOptimistic(
 		if bytesutil.IsHex(stateId) {
 			id, err := hexutil.Decode(stateIdString)
 			if err != nil {
-				return false, err
+				e := lookup.NewStateIdParseError(err)
+				return false, &e
 			}
 			return isStateRootOptimistic(ctx, id, optimisticModeFetcher, stateFetcher, chainInfo, database)
 		} else if len(stateId) == 32 {
@@ -127,7 +128,7 @@ func isStateRootOptimistic(
 ) (bool, error) {
 	st, err := stateFetcher.State(ctx, stateId)
 	if err != nil {
-		return true, errors.Wrap(err, "could not fetch state")
+		return true, lookup.NewFetchStateError(err)
 	}
 	if st.Slot() == chainInfo.HeadSlot() {
 		return optimisticModeFetcher.IsOptimistic(ctx)
@@ -137,7 +138,7 @@ func isStateRootOptimistic(
 		return true, errors.Wrapf(err, "could not get block roots for slot %d", st.Slot())
 	}
 	if !has {
-		return true, errors.New("no block roots returned from the database")
+		return true, lookup.NewBlockRootsNotFoundError()
 	}
 	for _, r := range roots {
 		b, err := database.Block(ctx, r)
diff --git a/beacon-chain/rpc/eth/helpers/sync_test.go b/beacon-chain/rpc/eth/helpers/sync_test.go
index ca007c0c53..d9799b0a05 100644
--- a/beacon-chain/rpc/eth/helpers/sync_test.go
+++ b/beacon-chain/rpc/eth/helpers/sync_test.go
@@ -1,12 +1,15 @@
 package helpers
 
 import (
+	"net/http"
+	"net/http/httptest"
 	"strconv"
 	"testing"
 
 	chainmock "github.com/OffchainLabs/prysm/v6/beacon-chain/blockchain/testing"
 	dbtest "github.com/OffchainLabs/prysm/v6/beacon-chain/db/testing"
 	doublylinkedtree "github.com/OffchainLabs/prysm/v6/beacon-chain/forkchoice/doubly-linked-tree"
+	"github.com/OffchainLabs/prysm/v6/beacon-chain/rpc/lookup"
 	"github.com/OffchainLabs/prysm/v6/beacon-chain/rpc/testutil"
 	"github.com/OffchainLabs/prysm/v6/beacon-chain/state"
 	state_native "github.com/OffchainLabs/prysm/v6/beacon-chain/state/state-native"
@@ -21,6 +24,7 @@ import (
 	"github.com/OffchainLabs/prysm/v6/testing/require"
 	"github.com/OffchainLabs/prysm/v6/testing/util"
 	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/pkg/errors"
 )
 
 func TestIsOptimistic(t *testing.T) {
@@ -226,7 +230,67 @@ func TestIsOptimistic(t *testing.T) {
 			require.NoError(t, err)
 			assert.Equal(t, true, o)
 		})
+		t.Run("State not found", func(t *testing.T) {
+			b, err := blocks.NewSignedBeaconBlock(util.NewBeaconBlock())
+			require.NoError(t, err)
+			b.SetStateRoot(bytesutil.PadTo([]byte("root"), 32))
+			db := dbtest.SetupDB(t)
+			require.NoError(t, db.SaveBlock(ctx, b))
+			chainSt, err := util.NewBeaconState()
+			require.NoError(t, err)
+			require.NoError(t, chainSt.SetSlot(fieldparams.SlotsPerEpoch))
+			bRoot, err := b.Block().HashTreeRoot()
+			require.NoError(t, err)
+			cs := &chainmock.ChainService{State: chainSt, OptimisticRoots: map[[32]byte]bool{bRoot: true}}
+			mf := &testutil.MockStater{
+				CustomError: lookup.NewFetchStateError(nil),
+			}
+			_, err = IsOptimistic(ctx, []byte(hexutil.Encode(bytesutil.PadTo([]byte("root"), 32))), cs, mf, cs, db)
+			var fetchErr *lookup.FetchStateError
+			require.Equal(t, true, errors.As(err, &fetchErr))
+		})
+
+		t.Run("stateId invalid", func(t *testing.T) {
+			b, err := blocks.NewSignedBeaconBlock(util.NewBeaconBlock())
+			require.NoError(t, err)
+			b.SetStateRoot(bytesutil.PadTo([]byte("root"), 32))
+			db := dbtest.SetupDB(t)
+			require.NoError(t, db.SaveBlock(ctx, b))
+			chainSt, err := util.NewBeaconState()
+			require.NoError(t, err)
+			require.NoError(t, chainSt.SetSlot(fieldparams.SlotsPerEpoch))
+			bRoot, err := b.Block().HashTreeRoot()
+			require.NoError(t, err)
+			cs := &chainmock.ChainService{State: chainSt, OptimisticRoots: map[[32]byte]bool{bRoot: true}}
+			mf := &testutil.MockStater{
+				CustomError: lookup.NewFetchStateError(nil),
+			}
+			_, err = IsOptimistic(ctx, []byte("0xabc"), cs, mf, cs, db)
+			var fetchErr *lookup.FetchStateError
+			require.Equal(t, false, errors.As(err, &fetchErr))
+		})
+		t.Run("block roots not found", func(t *testing.T) {
+			b, err := blocks.NewSignedBeaconBlock(util.NewBeaconBlock())
+			require.NoError(t, err)
+			b.SetStateRoot(bytesutil.PadTo([]byte("root"), 32))
+			db := dbtest.SetupDB(t)
+			require.NoError(t, db.SaveBlock(ctx, b))
+			chainSt, err := util.NewBeaconState()
+			require.NoError(t, err)
+			require.NoError(t, chainSt.SetSlot(fieldparams.SlotsPerEpoch))
+			bRoot, err := b.Block().HashTreeRoot()
+			require.NoError(t, err)
+			cs := &chainmock.ChainService{State: chainSt, OptimisticRoots: map[[32]byte]bool{bRoot: true}}
+			st, err := util.NewBeaconState()
+			require.NoError(t, st.SetSlot(primitives.Slot(fieldparams.SlotsPerEpoch+1)))
+			require.NoError(t, err)
+			mf := &testutil.MockStater{BeaconState: st}
+			_, err = IsOptimistic(ctx, []byte(hexutil.Encode(bytesutil.PadTo([]byte("root"), 32))), cs, mf, cs, db)
+			var blockRootsNotFoundErr *lookup.BlockRootsNotFoundError
+			require.Equal(t, true, errors.As(err, &blockRootsNotFoundErr))
+		})
 	})
+
 	t.Run("slot", func(t *testing.T) {
 		t.Run("head is not optimistic", func(t *testing.T) {
 			cs := &chainmock.ChainService{Optimistic: false}
@@ -319,6 +383,36 @@ func TestIsOptimistic(t *testing.T) {
 	})
 }
 
+func TestHandleIsOptimisticError(t *testing.T) {
+	t.Run("fetch-state error handled as 404", func(t *testing.T) {
+		rr := httptest.NewRecorder()
+		notFoundErr := lookup.StateNotFoundError{}
+		fetchErr := lookup.NewFetchStateError(&notFoundErr)
+		HandleIsOptimisticError(rr, fetchErr)
+
+		require.Equal(t, http.StatusNotFound, rr.Code)
+		require.StringContains(t, notFoundErr.Error(), rr.Body.String())
+	})
+	t.Run("no block roots error handled as 404", func(t *testing.T) {
+		rr := httptest.NewRecorder()
+		blockRootsErr := lookup.NewBlockRootsNotFoundError()
+		HandleIsOptimisticError(rr, blockRootsErr)
+
+		require.Equal(t, http.StatusNotFound, rr.Code)
+		require.StringContains(t, blockRootsErr.Error(), rr.Body.String())
+	})
+
+	t.Run("generic error handled as 500", func(t *testing.T) {
+		rr := httptest.NewRecorder()
+		genericErr := errors.New("boom")
+
+		HandleIsOptimisticError(rr, genericErr)
+
+		require.Equal(t, http.StatusInternalServerError, rr.Code)
+		require.StringContains(t, "Could not check optimistic status: boom", rr.Body.String())
+	})
+}
+
 // prepareForkchoiceState prepares a beacon state with the given data to mock
 // insert into forkchoice
 func prepareForkchoiceState(
diff --git a/beacon-chain/rpc/lookup/blocker.go b/beacon-chain/rpc/lookup/blocker.go
index fd5cef9bca..1a77d0d93b 100644
--- a/beacon-chain/rpc/lookup/blocker.go
+++ b/beacon-chain/rpc/lookup/blocker.go
@@ -23,6 +23,20 @@ import (
 	"github.com/pkg/errors"
 )
 
+type BlockRootsNotFoundError struct {
+	message string
+}
+
+func NewBlockRootsNotFoundError() *BlockRootsNotFoundError {
+	return &BlockRootsNotFoundError{
+		message: "no block roots returned from the database",
+	}
+}
+
+func (e BlockRootsNotFoundError) Error() string {
+	return e.message
+}
+
 // BlockIdParseError represents an error scenario where a block ID could not be parsed.
 type BlockIdParseError struct {
 	message string
diff --git a/beacon-chain/rpc/lookup/stater.go b/beacon-chain/rpc/lookup/stater.go
index 81d12c0b1b..abecc7a8d0 100644
--- a/beacon-chain/rpc/lookup/stater.go
+++ b/beacon-chain/rpc/lookup/stater.go
@@ -21,6 +21,27 @@ import (
 	"github.com/pkg/errors"
 )
 
+type FetchStateError struct {
+	message string
+	cause   error
+}
+
+func NewFetchStateError(cause error) *FetchStateError {
+	return &FetchStateError{
+		message: "could not fetch state",
+		cause:   cause,
+	}
+}
+
+func (e *FetchStateError) Error() string {
+	if e.cause != nil {
+		return e.message + ": " + e.cause.Error()
+	}
+	return e.message
+}
+
+func (e *FetchStateError) Unwrap() error { return e.cause }
+
 // StateIdParseError represents an error scenario where a state ID could not be parsed.
 type StateIdParseError struct {
 	message string
diff --git a/beacon-chain/rpc/prysm/beacon/validator_count.go b/beacon-chain/rpc/prysm/beacon/validator_count.go
index 545ad06bbd..7f0f71a0f3 100644
--- a/beacon-chain/rpc/prysm/beacon/validator_count.go
+++ b/beacon-chain/rpc/prysm/beacon/validator_count.go
@@ -56,11 +56,7 @@ func (s *Server) GetValidatorCount(w http.ResponseWriter, r *http.Request) {
 
 	isOptimistic, err := helpers.IsOptimistic(ctx, []byte(stateID), s.OptimisticModeFetcher, s.Stater, s.ChainInfoFetcher, s.BeaconDB)
 	if err != nil {
-		errJson := &httputil.DefaultJsonError{
-			Message: fmt.Sprintf("could not check if slot's block is optimistic: %v", err),
-			Code:    http.StatusInternalServerError,
-		}
-		httputil.WriteError(w, errJson)
+		helpers.HandleIsOptimisticError(w, err)
 		return
 	}
 
diff --git a/beacon-chain/rpc/testutil/mock_stater.go b/beacon-chain/rpc/testutil/mock_stater.go
index b0db3ae222..bd0949ba69 100644
--- a/beacon-chain/rpc/testutil/mock_stater.go
+++ b/beacon-chain/rpc/testutil/mock_stater.go
@@ -15,10 +15,14 @@ type MockStater struct {
 	BeaconStateRoot   []byte
 	StatesBySlot      map[primitives.Slot]state.BeaconState
 	StatesByRoot      map[[32]byte]state.BeaconState
+	CustomError       error
 }
 
 // State --
 func (m *MockStater) State(ctx context.Context, id []byte) (state.BeaconState, error) {
+	if m.CustomError != nil {
+		return nil, m.CustomError
+	}
 	if m.StateProviderFunc != nil {
 		return m.StateProviderFunc(ctx, id)
 	}
diff --git a/changelog/muzry_update_not_found_status.md b/changelog/muzry_update_not_found_status.md
new file mode 100644
index 0000000000..73b414add8
--- /dev/null
+++ b/changelog/muzry_update_not_found_status.md
@@ -0,0 +1,2 @@
+### Fixed
+- Fixed endpoint response to return 404 or 400 after isOptimistic check
\ No newline at end of file