diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 6bff0f705..0592919e4 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -967,11 +967,19 @@ def generate_seed(state: BeaconState,
 
 ```python
 def get_beacon_proposer_index(state: BeaconState,
-                              slot: Slot) -> ValidatorIndex:
+                              slot: Slot,
+                              registry_change: bool=False)) -> ValidatorIndex:
     """
     Return the beacon proposer index for the ``slot``.
     """
-    first_committee, _ = get_crosslink_committees_at_slot(state, slot)[0]
+    epoch = slot_to_epoch(slot)
+    current_epoch = get_current_epoch(state)
+    previous_epoch = get_previous_epoch(state)
+    next_epoch = current_epoch + 1
+
+    assert previous_epoch <= epoch <= next_epoch
+
+    first_committee, _ = get_crosslink_committees_at_slot(state, slot, registry_change)[0]
     return first_committee[slot % len(first_committee)]
 ```
 
diff --git a/specs/validator/0_beacon-chain-validator.md b/specs/validator/0_beacon-chain-validator.md
index 1523fa507..949a5e429 100644
--- a/specs/validator/0_beacon-chain-validator.md
+++ b/specs/validator/0_beacon-chain-validator.md
@@ -371,8 +371,7 @@ def get_committee_assignment(
         if len(selected_committees) > 0:
             validators = selected_committees[0][0]
             shard = selected_committees[0][1]
-            first_committee_at_slot = crosslink_committees[0][0]  # List[ValidatorIndex]
-            is_proposer = first_committee_at_slot[slot % len(first_committee_at_slot)] == validator_index
+            is_proposer = validator_index == get_beacon_proposer_index(state, slot, registry_change)
 
             assignment = (validators, shard, slot, is_proposer)
             return assignment