diff --git a/specs/phase1/shard-fork-choice.md b/specs/phase1/shard-fork-choice.md
index 1a449e3e9..b6d5c5ad4 100644
--- a/specs/phase1/shard-fork-choice.md
+++ b/specs/phase1/shard-fork-choice.md
@@ -170,7 +170,7 @@ def on_shard_block(store: Store, shard_store: ShardStore, signed_shard_block: Si
 
     # Check the block is valid and compute the post-state
     shard_state = shard_parent_state.copy()
-    shard_state_transition(beacon_parent_state, shard_state, shard_block)
+    shard_state_transition(shard_state, shard_block, validate_message=True, beacon_parent_state=beacon_parent_state)
 
     # Add new block to the store
     shard_store.blocks[hash_tree_root(shard_block)] = shard_block
diff --git a/specs/phase1/shard-transition.md b/specs/phase1/shard-transition.md
index 1433b0837..d725cd8bf 100644
--- a/specs/phase1/shard-transition.md
+++ b/specs/phase1/shard-transition.md
@@ -61,11 +61,12 @@ def verify_shard_block_signature(beacon_state: BeaconState,
 ## Shard state transition
 
 ```python
-def shard_state_transition(beacon_parent_state: BeaconState,
-                           shard_state: ShardState,
+def shard_state_transition(shard_state: ShardState,
                            block: ShardBlock,
-                           validate_message: bool = True) -> bool:
+                           validate_message: bool = True,
+                           beacon_parent_state: Optional[BeaconState] = None) -> bool:
     if validate_message:
+        assert beacon_parent_state is not None
         assert verify_shard_block_message(beacon_parent_state, shard_state, block)
 
     process_shard_block(shard_state, block)
@@ -126,7 +127,7 @@ def is_valid_fraud_proof(beacon_state: BeaconState,
     else:
         shard_state = transition.shard_states[offset_index - 1]  # Not doing the actual state updates here.
 
-    shard_state_transition(beacon_state, shard_state, block, validate_message=False)
+    shard_state_transition(shard_state, block, validate_message=False)
     if shard_state != transition.shard_states[offset_index]:
         return True
 
diff --git a/specs/phase1/validator.md b/specs/phase1/validator.md
index 39ada7891..d78726bc2 100644
--- a/specs/phase1/validator.md
+++ b/specs/phase1/validator.md
@@ -280,13 +280,12 @@ def get_shard_transition_fields(
     beacon_state: BeaconState,
     shard: Shard,
     shard_blocks: Sequence[SignedShardBlock],
-    validate_signature: bool=True,
 ) -> Tuple[Sequence[uint64], Sequence[Root], Sequence[ShardState]]:
     shard_states = []
     shard_data_roots = []
     shard_block_lengths = []
 
-    shard_state = beacon_state.shard_states[shard].copy()
+    shard_state = beacon_state.shard_states[shard]
     shard_block_slots = [shard_block.message.slot for shard_block in shard_blocks]
     offset_slots = compute_offset_slots(
         get_latest_slot_for_shard(beacon_state, shard),
@@ -299,7 +298,8 @@ def get_shard_transition_fields(
         else:
             shard_block = SignedShardBlock(message=ShardBlock(slot=slot, shard=shard))
             shard_data_roots.append(Root())
-        shard_state_transition(beacon_state, shard_state, shard_block.message, validate_message=False)
+        shard_state = shard_state.copy()
+        shard_state_transition(shard_state, shard_block.message, validate_message=False)
         shard_states.append(shard_state)
         shard_block_lengths.append(len(shard_block.message.body))