Output only two prong - no ground.

I bought this to dim outdoor LED string lights. Two star deduction because the female output is to a two-pronged plug - where as string lights are 3 prong with a ground. The mail in that plugs into the socket is 3 prong - where does the ground go? One of the pictures does show this - my bad for not noticing - it should be noted in the tech specs. Buyer beware.

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