diff --git a/specs/core/0_beacon-chain.md b/specs/core/0_beacon-chain.md
index c38b237bc..bc9e9bcfc 100644
--- a/specs/core/0_beacon-chain.md
+++ b/specs/core/0_beacon-chain.md
@@ -471,7 +471,7 @@ The following data structures are defined as [SimpleSerialize (SSZ)](https://git
 ```python
 {
     # Header
-    'slot': Slot,
+    'slot': 'uint64',
     'parent_root': 'bytes32',
     'state_root': 'bytes32',
     'randao_reveal': 'bytes96',