Command Line Build of Web Projects

I'm trying to build a number of components through the devenv command line using a solution. All projects are in the solution however I can not build the one web project I include.

The command I'm using is:
devenv /build debug /project webproject.csproj webproject.sln

If I open the solution then the project will build but on the command line I get the error 'invalid project'.

I've tried a number of web projects and they all show the same behaviour, but the command line will work for a local project.

Any thoughts on what the correct command would be?

Thanks, Celia.