diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index d934ccc98..fa6ae338b 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -902,7 +902,8 @@ def get_block_root(state: BeaconState,
     """
     Returns the block root at a recent ``slot``.
     """
-    assert slot > state.slot - LATEST_BLOCK_ROOTS_LENGTH
+    assert state.slot <= slot + LATEST_BLOCK_ROOTS_LENGTH
+    assert slot < state.slot
     return state.latest_block_roots[slot % LATEST_BLOCK_ROOTS_LENGTH]
 ```