has invalid or missing guid for metadata 'Project'.

Hello,

I have setup a Xamarin Forms project in Jenkins and our build is failing on what appears to be the PCL project.
The build gets all the way up to -> Target AssignProjectConfiguration:
/Library/Frameworks/Mono.framework/Versions/4.2.4/lib/mono/4.5/Microsoft.Common.targets: error : Project reference '../ProjectName/ProjectName.csproj' has invalid or missing guid for metadata 'Project'.

Thanks,
Grant