Re-fueling issue

Hello Everyone,

Just put my dart on the road this weekend and am trying to work out a few things.

I went to the gas station and here in Oregon there is no self service. I asked for a fill, and the attendant locked the handle so it would fill automatically. Once it clicked off, fuel shot back out of the filler neck and they had to wipe some off the quarter panel.

What would cause this? I checked my cap and it says "non vented" Is this correct? It is a "factory style" screw on, not a locking one.

Thanks!