This is why you should never fall in love with your costar.

Days after Us Weekly broke the news that Taylor Lautner and Lily Collins ended their one-year romance, the Abduction actors reunited to promote the thriller in London Monday.

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Though they posed side-by-side, neither Lautner, 19, nor Collins, 22, cracked a smile for photographers during the red carpet reunion.

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And while Lautner has raved about the chemistry they once shared, audiences weren't nearly as enamored with the two actors: Abduction debuted with a disappointing $11.2 million in its opening weekend.

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What caused the young costars to split just one week before the film's L.A. premiere?

"It was a clean break that Taylor initiated," a source told Us, noting that they "barely said a word to one another" at the Abduction afterparty in L.A. September 15.

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Despite his newly single status, the source added that Lautner isn't ready for another romance just yet: "He wants to be single for awhile."

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