diff --git a/scripts/build_spec.py b/scripts/build_spec.py
index 114832bc2a..16dd0d21d1 100644
--- a/scripts/build_spec.py
+++ b/scripts/build_spec.py
@@ -31,7 +31,7 @@
 SSZObject = TypeVar('SSZObject', bound=SSZType)
 '''
 PHASE1_IMPORTS = '''from typing import (
-    Any, Dict, Set, Sequence, MutableSequence, NewType, Tuple, Union, TypeVar
+    Any, Dict, Set, Sequence, MutableSequence, NewType, Optional, Tuple, Union, TypeVar
 )
 from math import (
     log2,
diff --git a/specs/phase0/beacon-chain.md b/specs/phase0/beacon-chain.md
index dcc03fc473..8a33461f19 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: Optional[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)
 ```