diff --git a/mesonbuild/environment.py b/mesonbuild/environment.py
index e29224b5ca00..4168730ffcd2 100644
--- a/mesonbuild/environment.py
+++ b/mesonbuild/environment.py
@@ -700,7 +700,7 @@ def __init__(self, source_dir: str, build_dir: str, cmd_options: coredata.Shared
         # Store a global state of Cargo dependencies
         self.cargo: T.Optional[cargo.Interpreter] = None
 
-    def mfilestr2key(self, machine_file_string: str, section_subproject: str, machine: MachineChoice) -> OptionKey:
+    def mfilestr2key(self, machine_file_string: str, section_subproject: T.Optional[str], machine: MachineChoice) -> OptionKey:
         key = OptionKey.from_string(machine_file_string)
         assert key.machine == MachineChoice.HOST
         if key.subproject: