diff --git a/src/utils/parse/spec.py b/src/utils/parse/spec.py
index e03c8e23..442656e4 100644
--- a/src/utils/parse/spec.py
+++ b/src/utils/parse/spec.py
@@ -23,7 +23,7 @@ class SpecArgumentType(object):
         self._set_modifier(modifier)
         self.exported = exported
 
-    def _set_modifier(self, modifier: str):
+    def _set_modifier(self, modifier: typing.Optional[str]):
         pass
 
     def name(self) -> typing.Optional[str]: