diff --git a/src/proto/mbyte.pro b/src/proto/mbyte.pro
index 806a6c6..83bcadc 100644
--- a/src/proto/mbyte.pro
+++ b/src/proto/mbyte.pro
@@ -40,6 +40,7 @@
 int utf_iscomposing(int c);
 int utf_printable(int c);
 int utf_class(int c);
+int utf_class_buf(int c, buf_T *buf);
 int utf_ambiguous_width(int c);
 int utf_fold(int a);
 int utf_toupper(int a);