diff --git a/src/proto/testing.pro b/src/proto/testing.pro
index 7542ccb..b812c2a 100644
--- a/src/proto/testing.pro
+++ b/src/proto/testing.pro
@@ -26,6 +26,7 @@
 void f_test_null_dict(typval_T *argvars, typval_T *rettv);
 void f_test_null_job(typval_T *argvars, typval_T *rettv);
 void f_test_null_list(typval_T *argvars, typval_T *rettv);
+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_unknown(typval_T *argvars, typval_T *rettv);