diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index b5e36bcf0..f372c457a 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -786,7 +786,7 @@ def generate_seed(state: BeaconState,
     Generate a seed for the given ``epoch``.
     """
     return hash(
-        get_randao_mix(state, Epoch(epoch + EPOCHS_PER_HISTORICAL_VECTOR - MIN_SEED_LOOKAHEAD)) +
+        get_randao_mix(state, Epoch(epoch + EPOCHS_PER_HISTORICAL_VECTOR - MIN_SEED_LOOKAHEAD)) +  # Avoid underflow
         hash_tree_root(List[ValidatorIndex, VALIDATOR_REGISTRY_LIMIT](get_active_validator_indices(state, epoch))) +
         int_to_bytes(epoch, length=32)
     )