Martin nervous about Rihanna hook-up

Chris Martin was scared to ask Rihanna to sing on Coldplay's new album.

The singer - who is married to Hollywood actress Gwyneth Paltrow - had to "pluck up the courage" to ask the 'Only Girl (In The World)' hitmaker to appear as a guest vocalist on track 'Princess Of China' from new album 'Mylo Xyloto'.