diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 25b1e91404d..3ee86d20eed 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -2684,7 +2684,7 @@ def name_and_dependency_types(s: str) -> Tuple[str, dt.DepFlag]:
                 return name, depflag
 
             def spec_and_dependency_types(
-                s: Union[Spec, Tuple[Spec, str]]
+                s: Union[Spec, Tuple[Spec, str]],
             ) -> Tuple[Spec, dt.DepFlag]:
                 """Given a non-string key in the literal, extracts the spec
                 and its dependency types.