diff --git a/src/proto/spell.pro b/src/proto/spell.pro
index 6b1f84b..3f97008 100644
--- a/src/proto/spell.pro
+++ b/src/proto/spell.pro
@@ -27,7 +27,7 @@
 void init_spell_chartab(void);
 int spell_iswordp(char_u *p, win_T *wp);
 int spell_iswordp_nmw(char_u *p, win_T *wp);
-int spell_casefold(char_u *str, int len, char_u *buf, int buflen);
+int spell_casefold(win_T *wp, char_u *str, int len, char_u *buf, int buflen);
 int check_need_cap(linenr_T lnum, colnr_T col);
 void ex_spellrepall(exarg_T *eap);
 void onecap_copy(char_u *word, char_u *wcopy, int upper);