Build tries to recompile source from referenced projects

[CoolDadTx]

I have 3 different NuBuild projects in a solution. All 3 reference library projects in the same solution. For the first 2 everything is fine. For the last one I get build errors related to source code from one of the referenced projects. This might be happening for all 3 projects but I only get compilation errors in one of them. Looking at the build log the Sources property for the project is getting set to the list of source files from one of the referenced library projects (not all of them). This particular set of source files have a dependency on an assembly that isn't included in a default C# project but is part of the framework (System.ComponentModel.DataAnnotations). I cannot figure why the NuBuild project would be trying to compile the source file for a referenced project. Nevertheless this is causing a build failure.