David Foster's daughter Amy thinks he and his fiancée Katharine Mcphee are ''perfect for each other''.

The 68-year-old musician is getting set to marry the 'Scorpion' actress, and after some critics slammed the relationship because Katharine is 34 years his junior, his 44-year-old daughter Amy - whom he has with first wife B.J. Cook - has leapt to their defence.

She said: ''I know that some people have been hating on that relationship. All I can tell you is it absolutely works when you see them together. It's love. The haters [can] shut the f**k up because they are perfect for each other.''

And Amy has nothing but kind words to say about the 34-year-old actress, who will become her step-mother when she ties the knot with David.

She added to Us Weekly magazine: ''I'm so close with [McPhee]. She's amazing and funny, fun, smart, elegant, timeless, beautiful, talented. I can't say enough about her.''

David - who also has Tim, 49, Allison, 48, Sara, 37, Erin, 35, and Jordan, 31, from previous relationships - popped the question to Katharine whilst the two were on vacation in Capri earlier this month.

It will be the fifth marriage for the 'You're The Inspiration' hitmaker, who was previously married to B.J. Cook, Rebecca Dyer, Linda Thomspon, and Yolanda Hadid.

Earlier this month, third wife Linda revealed she had concerns over the pair's large age gap.

She said: ''I wish them well. I think the only thing that's a real deterrent, I think, is the age difference.''

But sources have reported the couple have a ''special bond''.

They said: ''From the very beginning, he and Katharine had a special bond. David took a lot of time off work to spend with Katharine. He is dedicated to his work, but he didn't want to miss out on any time with Katharine, so it was worth it for him.''