Pop star Rita Ora announced at the weekend that she had been cast in the role of Christian Grey's younger adoptive sister in the Fifty Shades of Grey erotic trilogy adaptation. Ora tweeted the news from the British Fashion Awards but wasn't met with the overwhelmingly positive reaction she may have been hoping for.

Rita Ora
Rita Ora Has Been Cast As Christian Grey's Sister In '50 Shades.'

"It's official! I've been cast in @FiftyShades of Grey as Christian's sister Mia. #FiftyShades," tweeted Rita. Though many congratulated the 'Shine Ya Light' singer or her exciting new movie role, some even went as far as to call a hoax. "Please tell me it's a joke that Rita Ora has been cast in Fifty Shades of Grey," wrote one Danielle Joshua whilst "CrazyFangirl" added, "Rita Ora. Mia Grey. Are you serious? #No."

More: Watch the 50 Shades of Grey Trailer

So why have fans reacted so negatively to Rita's casting? Part of the problem could be to do with the perceived disparity between Rita's characteristics and the description laid out by author EL James in the novels. Though it's unlikely that every actor will fit James' description perfectly, at this early stage fans are eager to know that Sam Taylor-Johnson's movie is on the right track.

Jamie Dornan
Jamie Dornan Gears Up To Begin Filming The Steamy Movie This Weekend.

In the novels, Mia is depicted as an outgoing, cello playing, dark haired and curvaceous 22 year-old. Rita is 23; so far so good. Plus she has allegedly been kicked out of her flat for rowdiness so she's got the "outgoing" part down too. However, the sister of billionaire Christian Grey is also said to be one of the only markedly tall characters in the book and Ora reportedly measures up at a pretty average 5'5": that's even shorter than Dakota Johnson.

Many fans have pointed out that the Kosovo-born Rita is not Caucasian and so viewers may have a hard time believing she and Jamie Dornan's Grey are sisters. However, Mia is the adoptive sister of Christian and need not even slightly resemble Jamie. Though Ora is currently rocking a wavy, bleach blonde hair do she is naturally dark haired and with a quick snip of the scissors will be given the "sleek bob" James describes.

Dakota Johnson
Dakota Johnson Is Ready To Play Anastasia Steele.

The movie could have done a lot worse than Rita Ora: she is a known name so will bring additional interest towards the film and she had has experience of acting, having appeared in 2004's Spivs, an episode of 90210 and this year's Fast & Furious 6.

Fifty Shades of Grey will begin filming in Vancouver this weekend.

More: Watch the Fifty Shades of Grey Trailer