diff --git a/strbuf.h b/strbuf.h
index 39c14cf..6b6f745 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -45,6 +45,7 @@
 extern void strbuf_trim(struct strbuf *);
 extern void strbuf_rtrim(struct strbuf *);
 extern void strbuf_ltrim(struct strbuf *);
+extern void strbuf_tolower(struct strbuf *sb);
 extern int strbuf_cmp(const struct strbuf *, const struct strbuf *);
 
 /*