diff --git a/lib/packet_datatypes.c b/lib/packet_datatypes.c
index 41dd9d94..335cb9f0 100644
--- a/lib/packet_datatypes.c
+++ b/lib/packet_datatypes.c
@@ -130,6 +130,7 @@ int packet__read_string(struct mosquitto__packet *packet, char **str, int *lengt
 	if(mosquitto_validate_utf8(*str, *length)){
 		mosquitto__free(*str);
 		*str = NULL;
+		*length = -1;
 		return MOSQ_ERR_MALFORMED_UTF8;
 	}