April 10, 2008 02:10 PM

The guests wore white – and Gwyneth Paltrow was no exception.

When Beyoncé Knowles wed longtime boyfriend Jay-Z in New York City April 4, good pal Paltrow donned a bone silk gazar minidress with full sleeves, which she borrowed from top designer Proenza Schouler.

“It was arranged last minute,” a rep for the designer tells PEOPLE, adding that the frock was a sample from the fall/winter 2008 collection and is not yet in stores.

Did the 35-year-old actress mention that she’d be wearing it to the top-secret nuptials (which have not yet been confirmed by the couple, although Knowles has been spotted wearing a ring)?

“No, we had no idea,” says the rep. “We just knew it was for a private event. No one knew about the wedding until day of.”

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