diff --git a/mesonbuild/interpreter/type_checking.py b/mesonbuild/interpreter/type_checking.py
index 7d5665c46..d4e178d12 100644
--- a/mesonbuild/interpreter/type_checking.py
+++ b/mesonbuild/interpreter/type_checking.py
@@ -11,7 +11,7 @@ from ..interpreterbase.decorators import KwargInfo, ContainerTypeInfo
 from ..mesonlib import FileMode, MachineChoice
 
 # Helper definition for type checks that are `Optional[T]`
-NoneType = type(None)
+NoneType: T.Type[None] = type(None)
 
 
 def in_set_validator(choices: T.Set[str]) -> T.Callable[[str], T.Optional[str]]: