Cannot build Lightswitch project after adding a datasource

Question

I am trying to add a sql database to my lightswitch application project however when I do and try to build or run the project I get .net errors such as 'type name system.datetime does not exist in lightswitchapplication.system' and other errors that are similar.

When I do remove the datasource and build the project it builds successfully.Why could it be that my existing datasources do not throw errors?

thanks