Demi Lovato is expanding her repertoire ... to Lil Wayne songs!

The young pop star hit the stage this weekend at Hammerstein Ballroom in NYC, where she performed a stripped-down version of Weezy's song "How To Love."

Demi sounded pretty good, but committed the ultimate cover sin: She forgot the lyrics to the song!

Check out her flub above -- and watch the full performance over on YouTube.

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