diff --git a/lib/spack/spack/directives.py b/lib/spack/spack/directives.py
index 2cff2678a04..3036d433d9e 100644
--- a/lib/spack/spack/directives.py
+++ b/lib/spack/spack/directives.py
@@ -610,7 +610,7 @@ def _execute_patch(
     return _execute_patch
 
 
-def conditional(*values: List[Any], when: Optional[WhenType] = None):
+def conditional(*values: Union[str, bool], when: Optional[WhenType] = None):
     """Conditional values that can be used in variant declarations."""
     # _make_when_spec returns None when the condition is statically false.
     when = _make_when_spec(when)