The What Makes You Beautiful singer has written tracks with a number of musicians, but the All About That Bass singer is the first to land a duet with Styles, and she confesses she was surprised by his writing skills on their collaboration Someday Maybe.
She tells Britain's The Sun newspaper, "It's a duet. It's a beautiful song we wrote together on my ukelele. We even recorded a rough demo together with our vocals together (sic). It's a cute acoustic love thing.
"It's one of the first cool things I got do with a pop star... A friend of mine said Harry was a fan and said he would love to write with me... He's an incredible writer, too. I was worried he'd just be a 20-year-old boy. So I walked in with a prepared verse and chorus. But he gave me very poetic, mature lyrics... I was like, 'Well, damn, Harry, you know what love is.'"
It is known not if the stars plan to release the song.