Drz 400 spark plug removal HELP

I recently purchased a drz400 doing a full service on it. I can't for the life of me get the spark plug out it should be a 16mm socket (5/8) is what the interent says and doesnt seem to fit it. With the new sparkplug I tried a 18mm socket and it fits the new spark plug with an 18mm but 18mm doesn't fit into the spark plug hole. Help I've tried many things and can't figure it out.