diff --git a/src/proto/charset.pro b/src/proto/charset.pro
index a747319..2f64077 100644
--- a/src/proto/charset.pro
+++ b/src/proto/charset.pro
@@ -16,6 +16,7 @@
 int vim_strsize(char_u *s);
 int vim_strnsize(char_u *s, int len);
 int chartabsize(char_u *p, colnr_T col);
+int win_chartabsize(win_T *wp, char_u *p, colnr_T col);
 int linetabsize_str(char_u *s);
 int linetabsize_col(int startcol, char_u *s);
 int win_linetabsize(win_T *wp, linenr_T lnum, char_u *line, colnr_T len);