Why do I get error 'File does not contain a valid CIL image' when debugging from Visual Studio Code editor?

I'm running Visual Studio Code on Max OS X and constantly seeing this error. Works fine when I run directly from Terminal using dnx .run

Any suggestions on how I can fix this?

 Error Trace from Terminal   'mono' --debug  
 --debugger-agent=transport=dt_socket,server=y,address=127.0.0.1:57332  
 'Program.cs' Cannot open assembly 'Program.cs': File does not contain
 a valid CIL image.