Do you really have to have had a kid to be a surrogate?

I have no plans to have children of my own anytime soon, but have been told repeatedly by doctors that I have a really great uterus, perfect in fact according to one. My family has smooth uncomplicated pregnancies.

I figured I could help families in need by becoming a surrogate but it seems everyone I ask (not agencies, just friends I discussed with) is saying they don't think I can because I have not been pregnant before.