diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 2827639af..4666d7dbf 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -1690,8 +1690,15 @@ For each `attestation` in `block.body.attestations`:
 ```python
     assert attestation.custody_bitfield == b'\x00' * len(attestation.custody_bitfield)  # [TO BE REMOVED IN PHASE 1]
     assert attestation.aggregation_bitfield != b'\x00' * len(attestation.aggregation_bitfield)
-
-    for i in range(len(crosslink_committee)):
+    
+    crosslink_committee = [
+        committee for committee, shard in get_crosslink_committees_at_slot(state, attestation.data.slot)
+        if shard == attestation.data.shard
+    ][0]
+    verify_bitfield(attestation.aggregation_bitfield, len(crosslink_committee))
+    verify_bitfield(attestation.custody_bitfield, len(crosslink_committee))
+    
+    for i in range(len(crosslink_committee):
         if get_bitfield_bit(attestation.aggregation_bitfield, i) == 0b0:
             assert get_bitfield_bit(attestation.custody_bitfield, i) == 0b0