Hanging out with the Queen has rubbed off on Niall Horan.

The singer attended a birthday party for The Saturdays' Rochelle Humes over the weekend, which appeared to be a costume party, since the One Direction member was dressed in a white tux a la Prince Charming. However, fans were miffed that he posed romantically with a princess – former 'X Factor' judge and singer Tulisa Contostavlos.

Horan cut a dashing figure at the party, held on Saturday, March 29. And Tulisa was so pretty in her matching white gown.

Directioners weren't pleased and they started getting ugly with their tweets and responses to the pics that Contostavlos uploaded to her Instagram account. They were jealous that she got to play dress up and gaze into Niall's baby blues. While we understand their ire -- since who wouldn't wish they could stare intently at Niall while playing dress up -- they were totally hating on the singer in unnecessary fashion.

She posted: "As If my little muffin turned up as Prince Charming looooool #bruddafromanothermother." That comment suggests she and Niall are like brother and sister with their relationship, squashing any romance rumors.

However, Contostavlos, 25, also has a legal issue hanging over her pretty head. She will stand trial in July, facing charges of supplying cocaine to an undercover reporter. It's a bit of a mess.

But Horan seems to still be her pal.

See their prince and princess photos below.