diff --git a/usr.bin/mail/extern.h b/usr.bin/mail/extern.h
--- a/usr.bin/mail/extern.h
+++ b/usr.bin/mail/extern.h
@@ -153,7 +153,7 @@
 int	 isdir(char *);
 int	 isfileaddr(char *);
 int	 ishead(char *);
-int	 isign(const char *, struct ignoretab []);
+int	 isign(const char *, struct ignoretab [2]);
 int	 isprefix(const char *, const char *);
 void	 istrncpy(char *, const char *, size_t);
 const struct cmd *