Setting lod (just one) in with hammer.

I decided to try making a dif today so I have set up hammer and made a crappy little test structure and exported it and all but when I try to put it in game I get this error.

unable to read detail level 0 in interior resource

I assume this is a problem with my detail level but all the forums I have looked at about it only talk about multiple LOD s and I don't want to deal with that at the moment.