py: Fix function type: () -> (void).
diff --git a/py/objstr.c b/py/objstr.c
index cfe0ef1..1bb89dd 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -46,7 +46,7 @@
 mp_obj_t mp_obj_new_str_iterator(mp_obj_t str);
 STATIC mp_obj_t mp_obj_new_bytes_iterator(mp_obj_t str);
 STATIC NORETURN void bad_implicit_conversion(mp_obj_t self_in);
-STATIC NORETURN void arg_type_mixup();
+STATIC NORETURN void arg_type_mixup(void);
 
 /******************************************************************************/
 /* str                                                                        */
@@ -1957,7 +1957,7 @@
     }
 }
 
-STATIC void arg_type_mixup() {
+STATIC void arg_type_mixup(void) {
     nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError, "Can't mix str and bytes arguments"));
 }