diff --git a/specs/phase0/beacon-chain.md b/specs/phase0/beacon-chain.md
index 478a2daa12..7f2e873bfb 100644
--- a/specs/phase0/beacon-chain.md
+++ b/specs/phase0/beacon-chain.md
@@ -788,7 +788,7 @@ def compute_activation_exit_epoch(epoch: Epoch) -> Epoch:
 #### `compute_domain`
 
 ```python
-def compute_domain(domain_type: DomainType, fork_version: Version=None) -> Domain:
+def compute_domain(domain_type: DomainType, fork_version: Optional[Version]=None) -> Domain:
     """
     Return the domain for the ``domain_type`` and ``fork_version``.
     """