diff --git a/lib/libc/string/strerror.c b/lib/libc/string/strerror.c
index be3732d5b9e..7cd984ea48f 100644
--- a/lib/libc/string/strerror.c
+++ b/lib/libc/string/strerror.c
@@ -58,7 +58,7 @@ __FBSDID("$FreeBSD$");
  * statically linked binaries.
  */
 static void
-errstr(int num, char *uprefix, char *buf, size_t len)
+errstr(int num, const char *uprefix, char *buf, size_t len)
 {
 	char *t;
 	unsigned int uerr;