diff --git a/pynitrokey/nk3/admin_app.py b/pynitrokey/nk3/admin_app.py
index 662c4c02..8cc44999 100644
--- a/pynitrokey/nk3/admin_app.py
+++ b/pynitrokey/nk3/admin_app.py
@@ -188,7 +188,7 @@ def factory_reset(self) -> None:
                 raise e
         FactoryResetStatus.check(reply[0], "Failed to factory reset the device")
 
-    def factory_reset_app(self, application) -> None:
+    def factory_reset_app(self, application: str) -> None:
         reply = self._call(
             AdminCommand.FACTORY_RESET_APP,
             data=application.encode("ascii"),