diff --git a/api/client/builder/client.go b/api/client/builder/client.go
index faff61d729..cf6b13327a 100644
--- a/api/client/builder/client.go
+++ b/api/client/builder/client.go
@@ -274,9 +274,6 @@ func non200Err(response *http.Response) error {
 	if err != nil {
 		body = "(Unable to read response body.)"
 	} else {
-		if jsonErr := json.Unmarshal(bodyBytes, &errMessage); jsonErr != nil {
-			return errors.Wrap(jsonErr, "unable to read response body")
-		}
 		body = "response body:\n" + string(bodyBytes)
 	}
 	msg := fmt.Sprintf("code=%d, url=%s, body=%s", response.StatusCode, response.Request.URL, body)
@@ -285,13 +282,25 @@ func non200Err(response *http.Response) error {
 		log.WithError(ErrNoContent).Debug(msg)
 		return ErrNoContent
 	case 400:
+		if jsonErr := json.Unmarshal(bodyBytes, &errMessage); jsonErr != nil {
+			return errors.Wrap(jsonErr, "unable to read response body")
+		}
 		log.WithError(ErrBadRequest).Debug(msg)
 		return errors.Wrap(ErrBadRequest, errMessage.Message)
 	case 404:
+		if jsonErr := json.Unmarshal(bodyBytes, &errMessage); jsonErr != nil {
+			return errors.Wrap(jsonErr, "unable to read response body")
+		}
 		log.WithError(ErrNotFound).Debug(msg)
 		return errors.Wrap(ErrNotFound, errMessage.Message)
-	default:
+	case 500:
+		if jsonErr := json.Unmarshal(bodyBytes, &errMessage); jsonErr != nil {
+			return errors.Wrap(jsonErr, "unable to read response body")
+		}
 		log.WithError(ErrNotOK).Debug(msg)
 		return errors.Wrap(ErrNotOK, errMessage.Message)
+	default:
+		log.WithError(ErrNotOK).Debug(msg)
+		return errors.Wrap(ErrNotOK, fmt.Sprintf("unsupported error code: %d", response.StatusCode))
 	}
 }
diff --git a/api/client/builder/client_test.go b/api/client/builder/client_test.go
index 8210625d1d..9d711e0aaa 100644
--- a/api/client/builder/client_test.go
+++ b/api/client/builder/client_test.go
@@ -144,6 +144,23 @@ func TestClient_GetHeader(t *testing.T) {
 	_, err := c.GetHeader(ctx, slot, bytesutil.ToBytes32(parentHash), bytesutil.ToBytes48(pubkey))
 	require.ErrorIs(t, err, ErrNotOK)
 
+	hc = &http.Client{
+		Transport: roundtrip(func(r *http.Request) (*http.Response, error) {
+			require.Equal(t, expectedPath, r.URL.Path)
+			return &http.Response{
+				StatusCode: http.StatusNoContent,
+				Body:       io.NopCloser(bytes.NewBuffer([]byte("No header is available."))),
+				Request:    r.Clone(ctx),
+			}, nil
+		}),
+	}
+	c = &Client{
+		hc:      hc,
+		baseURL: &url.URL{Host: "localhost:3500", Scheme: "http"},
+	}
+	_, err = c.GetHeader(ctx, slot, bytesutil.ToBytes32(parentHash), bytesutil.ToBytes48(pubkey))
+	require.ErrorIs(t, err, ErrNoContent)
+
 	hc = &http.Client{
 		Transport: roundtrip(func(r *http.Request) (*http.Response, error) {
 			require.Equal(t, expectedPath, r.URL.Path)