diff --git a/openbsc/src/libmgcp/mgcp_protocol.c b/openbsc/src/libmgcp/mgcp_protocol.c
index dc5e0f9..d38bbf3 100644
--- a/openbsc/src/libmgcp/mgcp_protocol.c
+++ b/openbsc/src/libmgcp/mgcp_protocol.c
@@ -862,7 +862,7 @@
 {
 	int res = 0;
 	char *line;
-	char tone = 0;
+	char tone = CHAR_MAX;
 
 	if (p->found != 0)
 		return create_err_response(NULL, 400, "RQNT", p->trans);