diff --git a/lib/libpam/modules/pam_opie/pam_opie.c b/lib/libpam/modules/pam_opie/pam_opie.c
index bfb875f8831..9625373d9d3 100644
--- a/lib/libpam/modules/pam_opie/pam_opie.c
+++ b/lib/libpam/modules/pam_opie/pam_opie.c
@@ -62,7 +62,7 @@ pam_sm_authenticate(pam_handle_t *pamh, int flags __unused,
 	struct passwd *pwd;
 	int retval, i;
 	const char *(promptstr[]) = { "%s\nPassword: ", "%s\nPassword [echo on]: "};
-	char challenge[OPIE_CHALLENGE_MAX];
+	char challenge[OPIE_CHALLENGE_MAX + 1];
 	char principal[OPIE_PRINCIPAL_MAX];
 	const char *user;
 	char *response;