diff --git a/src/proto/mbyte.pro b/src/proto/mbyte.pro
index 7883b3b..c57c94c 100644
--- a/src/proto/mbyte.pro
+++ b/src/proto/mbyte.pro
@@ -85,6 +85,7 @@
 int convert_input_safe(char_u *ptr, int len, int maxlen, char_u **restp, int *restlenp);
 char_u *string_convert(vimconv_T *vcp, char_u *ptr, int *lenp);
 char_u *string_convert_ext(vimconv_T *vcp, char_u *ptr, int *lenp, int *unconvlenp);
+int get_cellwidth(int c);
 void f_setcellwidths(typval_T *argvars, typval_T *rettv);
 void f_getcellwidths(typval_T *argvars, typval_T *rettv);
 void f_charclass(typval_T *argvars, typval_T *rettv);