diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 70c5b67f6..9bec7841f 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -1160,9 +1160,9 @@ def verify_bitfield(bitfield: bytes, committee_size: int) -> bool:
 ### `convert_to_indexed`
 
 ```python
-def convert_to_indexed(state: BeaconState, attestation: Attestation):
+def convert_to_indexed(state: BeaconState, attestation: Attestation) -> IndexedAttestation:
     """
-    Convert an attestation to (almost) indexed-verifiable form
+    Convert ``attestation`` to (almost) indexed-verifiable form.
     """
     attesting_indices = get_attestation_participants(state, attestation.data, attestation.aggregation_bitfield)
     custody_bit_1_indices = get_attestation_participants(state, attestation.data, attestation.custody_bitfield)
@@ -1172,7 +1172,7 @@ def convert_to_indexed(state: BeaconState, attestation: Attestation):
         custody_bit_0_indices=custody_bit_0_indices,
         custody_bit_1_indices=custody_bit_1_indices,
         data=attestation.data,
-        aggregate_signature=attestation.aggregate_signature
+        aggregate_signature=attestation.aggregate_signature,
     )
 ```