diff --git a/mesonbuild/modules/fs.py b/mesonbuild/modules/fs.py
index d48311114..5ba1e3819 100644
--- a/mesonbuild/modules/fs.py
+++ b/mesonbuild/modules/fs.py
@@ -24,11 +24,11 @@ from ..interpreterbase import FeatureNew
 
 from ..interpreterbase import stringArgs, noKwargs
 if T.TYPE_CHECKING:
-    from ..interpreter import ModuleState
+    from ..interpreter import Interpreter, ModuleState
 
 class FSModule(ExtensionModule):
 
-    def __init__(self, interpreter):
+    def __init__(self, interpreter: 'Interpreter') -> None:
         super().__init__(interpreter)
         self.snippets.add('generate_dub_file')
 
@@ -36,7 +36,7 @@ class FSModule(ExtensionModule):
         """
         make an absolute path from a relative path, WITHOUT resolving symlinks
         """
-        return Path(state.source_root) / state.subdir / Path(arg).expanduser()
+        return Path(state.source_root) / Path(state.subdir) / Path(arg).expanduser()
 
     def _resolve_dir(self, state: 'ModuleState', arg: str) -> Path:
         """
@@ -193,5 +193,5 @@ class FSModule(ExtensionModule):
         new = original.stem
         return ModuleReturnValue(str(new), [])
 
-def initialize(*args, **kwargs) -> FSModule:
+def initialize(*args: T.Any, **kwargs: T.Any) -> FSModule:
     return FSModule(*args, **kwargs)
diff --git a/run_mypy.py b/run_mypy.py
index c7d44b606..3369b1a76 100755
--- a/run_mypy.py
+++ b/run_mypy.py
@@ -38,6 +38,7 @@ strict_modules = [
   'mesonbuild/ast',
   'mesonbuild/wrap',
   'mesonbuild/scripts',
+  'mesonbuild/modules/fs.py',
   'mesonbuild/dependencies/boost.py',
   'mesonbuild/dependencies/hdf5.py',
   'mesonbuild/compilers/mixins/intel.py',