How do you judge a singing competition when you don't sing live yourself? 

That's the question a lot of "American Idol" fans started asking on Twitter last night during Jennifer Lopez's performance of "I Luh Ya Papi."

While she looked amazing rocking some sparkly Daisy Dukes and dancing up a storm on stage, her vocals took a back seat -- and the lip-sync criticisms started flying online.

To be fair, J.Lo did start out the performance with a lovely a cappella verse, harmonizing with "Idol" alums Pia Toscano, Jessica Sanchez and Allison Iraheta before the backing track took over.

But the haterade began once she started dancing -- and even included a tweet from Travel Channel host Andrew Zimmern.

It's common for singers to let a track do the singing while they focus on the choreography -- but what did you think of the performance? Sound off below!


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