diff --git a/specs/altair/beacon-chain.md b/specs/altair/beacon-chain.md
index 5bce9ba4e..5a8dcce24 100644
--- a/specs/altair/beacon-chain.md
+++ b/specs/altair/beacon-chain.md
@@ -481,8 +481,8 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
     proposer_reward_numerator = 0
     for index in get_attesting_indices(state, data, attestation.aggregation_bits):
         for flag_index, weight in enumerate(PARTICIPATION_FLAG_WEIGHTS):
-            if flag_index in participation_flag_indices and not epoch_participation[index][flag_index]:
-                epoch_participation[index][flag_index] = True
+            if flag_index in participation_flag_indices and not has_flag(epoch_participation[index], flag_index):
+                epoch_participation[index] = add_flag(epoch_participation[index], flag_index)
                 proposer_reward_numerator += get_base_reward(state, index) * weight
 
     # Reward proposer