diff --git a/specs/phase1/beacon-chain.md b/specs/phase1/beacon-chain.md
index fccb93f55..4719d2e6f 100644
--- a/specs/phase1/beacon-chain.md
+++ b/specs/phase1/beacon-chain.md
@@ -537,9 +537,13 @@ def get_light_client_committee(beacon_state: BeaconState, epoch: Epoch) -> Seque
 
 ```python
 def get_shard_proposer_index(beacon_state: BeaconState, slot: Slot, shard: Shard) -> ValidatorIndex:
-    committee = get_shard_committee(beacon_state, compute_epoch_at_slot(slot), shard)
+    """
+    Return the proposer's index of shard block at ``slot``.
+    """
     epoch = compute_epoch_at_slot(slot)
-    r = bytes_to_int(get_seed(beacon_state, epoch, DOMAIN_SHARD_COMMITTEE)[:8])
+    committee = get_shard_committee(beacon_state, epoch, shard)
+    seed = hash(get_seed(beacon_state, epoch, DOMAIN_SHARD_COMMITTEE) + int_to_bytes(slot, length=8))
+    r = bytes_to_int(seed[:8])
     return committee[r % len(committee)]
 ```