call me DADDY

So my hubby has been hinting about wanting me to call him "daddy" during sex. I'm normally up for anything & wanna forfill his desires but for some reason i can't bring myself to say it ....I try & I just bite my tongue. I get so shy & embarresed by it...what should I do? tip? , .........I even played a game & bet saying I would call him daddy if I lost ....Well I lost & still couldnt.....