diff --git a/src/proto/diff.pro b/src/proto/diff.pro
index e45127a..f314fce 100644
--- a/src/proto/diff.pro
+++ b/src/proto/diff.pro
@@ -22,5 +22,6 @@
 void ex_diffgetput __ARGS((exarg_T *eap));
 int diff_mode_buf __ARGS((buf_T *buf));
 int diff_move_to __ARGS((int dir, long count));
+linenr_T diff_get_corresponding_line __ARGS((buf_T *buf1, linenr_T lnum1, buf_T *buf2, linenr_T lnum2));
 linenr_T diff_lnum_win __ARGS((linenr_T lnum, win_T *wp));
 /* vim: set ft=c : */