diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index 34f87523a..910d02972 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -747,6 +747,7 @@ def get_permuted_index(index: int, list_size: int, seed: Bytes32) -> int:
     See the 'generalized domain' algorithm on page 3.
     """
     assert index < list_size
+    assert list_size <= 2**40
     
     for round in range(SHUFFLE_ROUND_COUNT):
         pivot = bytes_to_int(hash(seed + int_to_bytes1(round))[0:8]) % list_size