"Build failed. Method not found: 'Int32'

Debugging UDK is a nightmare. So hoping Unity simplifies troubleshooting and offers interactive debugging. The problem is whenever I set a breakpoint on a Project in Monodevelop, I keep getting this error using Unity 4.5 / 4.6 :- •"Build failed. Method not found:

'Int32 System.Environment.get_CurrentManagedThreadId()'"

If there's some extra set-up required between Unity and Mondevelop I'd welcome some advice. I'm testing this on multiple machines and keep getting the same error. Cheers!