diff --git a/specs/phase0/beacon-chain.md b/specs/phase0/beacon-chain.md
index 503286e48..478a2daa1 100644
--- a/specs/phase0/beacon-chain.md
+++ b/specs/phase0/beacon-chain.md
@@ -788,10 +788,12 @@ def compute_activation_exit_epoch(epoch: Epoch) -> Epoch:
 #### `compute_domain`
 
 ```python
-def compute_domain(domain_type: DomainType, fork_version: Version=GENESIS_FORK_VERSION) -> Domain:
+def compute_domain(domain_type: DomainType, fork_version: Version=None) -> Domain:
     """
     Return the domain for the ``domain_type`` and ``fork_version``.
     """
+    if fork_version is None:
+        fork_version = GENESIS_FORK_VERSION
     return Domain(domain_type + fork_version)
 ```