diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index bf65b66c1..09f8b7280 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -1787,7 +1787,7 @@ def get_current_total_balance(state: BeaconState) -> Gwei:
 ```
 
 ```python
-def get_previous_total_balance(state: BeaconState):
+def get_previous_total_balance(state: BeaconState) -> Gwei:
     return get_total_balance(state, get_active_validator_indices(state.validator_registry, previous_epoch))
 ```