diff --git a/src/proto/testing.pro b/src/proto/testing.pro
index dea4f75..1c93f8c 100644
--- a/src/proto/testing.pro
+++ b/src/proto/testing.pro
@@ -30,6 +30,7 @@
 void f_test_null_function(typval_T *argvars, typval_T *rettv);
 void f_test_null_partial(typval_T *argvars, typval_T *rettv);
 void f_test_null_string(typval_T *argvars, typval_T *rettv);
+void f_test_null_tuple(typval_T *argvars, typval_T *rettv);
 void f_test_unknown(typval_T *argvars, typval_T *rettv);
 void f_test_void(typval_T *argvars, typval_T *rettv);
 void f_test_setmouse(typval_T *argvars, typval_T *rettv);