To not want to call MIL Mum?

[sunshinelolipops]

My MIL wants me to call her Mum and gets upset if I call her by her first name. I talked to Dh about it and he doesn't get the problem and thinks this is a normal thing to do. He is very defensive of his Mum.
I don't feel close to MIL and it feels awkward and weird to call her Mum. I am very close to my actual Mum. At the same time I don't want to upset her and have been humouring it but don't know how long I can keep it up for. I am also worried it might offend my actual mum if she heard me call her this.
My mum doesn't expect Dh to call her Mum, just by first name. I had never even heard of calling MILs Mum before.
Is this normal? What should I do?

Err I didn't say it doesn't happen . I said I hadn't heard of it before. After starting this thread I now know that some people do this. But the majority don't feel ok with it.

I’d just say, sorry I don’t feel comfortable calling anyone except my mum, mum. I’d prefer to continue using your first name.