diff --git a/tmux.h b/tmux.h
index 645e5003..04088a16 100644
--- a/tmux.h
+++ b/tmux.h
@@ -1521,7 +1521,7 @@ int		 session_last(struct session *);
 
 /* utf8.c */
 void	utf8_build(void);
-int	utf8_width(u_char *);
+int	utf8_width(const u_char *);
 
 /* util.c */
 char   *section_string(char *, size_t, size_t, size_t);
diff --git a/utf8.c b/utf8.c
index b0e3c3cb..9458d8b2 100644
--- a/utf8.c
+++ b/utf8.c
@@ -297,7 +297,7 @@ utf8_split(u_int uvalue, u_char *data)
 }
 
 int
-utf8_width(u_char *udata)
+utf8_width(const u_char *udata)
 {
 	struct utf8_width_entry	*item;
 	u_int			 uvalue;