diff --git a/medmysql.c b/medmysql.c
index 37dd30e..a5fcfb5 100644
--- a/medmysql.c
+++ b/medmysql.c
@@ -511,7 +511,7 @@ int medmysql_fetch_records(med_callid_t *callid,
 		callid->value, callid->value,
 		callid->value, callid->value);
 
-	assert(len < sizeof(query)); /* truncated - internal bug */
+	assert(len > 0 && (size_t)len < sizeof(query)); /* truncated - internal bug */
 
 	/*syslog(LOG_DEBUG, "q='%s'", query);*/