diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 079395c16..0fc48f090 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -971,11 +971,19 @@ 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..e1083c5c5 100644
--- a/specs/validator/0_beacon-chain-validator.md
+++ b/specs/validator/0_beacon-chain-validator.md
@@ -50,7 +50,7 @@ __NOTICE__: This document is a work-in-progress for researchers and implementers
                 - [Aggregation bitfield](#aggregation-bitfield)
                 - [Custody bitfield](#custody-bitfield)
                 - [Aggregate signature](#aggregate-signature)
-    - [Validator assigments](#validator-assignments)
+    - [Validator assignments](#validator-assignments)
         - [Lookahead](#lookahead)
     - [How to avoid slashing](#how-to-avoid-slashing)
         - [Proposer slashing](#proposer-slashing)
@@ -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=registry_change)
 
             assignment = (validators, shard, slot, is_proposer)
             return assignment