diff --git a/specs/phase1/beacon-chain.md b/specs/phase1/beacon-chain.md
index b64c31a57..6f40e5081 100644
--- a/specs/phase1/beacon-chain.md
+++ b/specs/phase1/beacon-chain.md
@@ -867,9 +867,10 @@ def apply_shard_transition(state: BeaconState, shard: Shard, transition: ShardTr
     # Verify combined proposer signature
     assert optional_aggregate_verify(pubkeys, signing_roots, transition.proposer_signature_aggregate)
 
-    # Save updated state
-    state.shard_states[shard] = transition.shard_states[len(transition.shard_states) - 1]
-    state.shard_states[shard].slot = compute_previous_slot(state.slot)
+    # Copy and save updated shard state
+    shard_state = copy(transition.shard_states[len(transition.shard_states) - 1])
+    shard_state.slot = compute_previous_slot(state.slot)
+    state.shard_states[shard] = shard_state
 ```
 
 ###### `process_crosslink_for_shard`