diff --git a/src/proto/diff.pro b/src/proto/diff.pro
index d14907e..9c34dcf 100644
--- a/src/proto/diff.pro
+++ b/src/proto/diff.pro
@@ -30,4 +30,5 @@
 linenr_T diff_lnum_win(linenr_T lnum, win_T *wp);
 void f_diff_filler(typval_T *argvars, typval_T *rettv);
 void f_diff_hlID(typval_T *argvars, typval_T *rettv);
+void f_diff(typval_T *argvars, typval_T *rettv);
 /* vim: set ft=c : */