Event add method '{0}' has return type '{1}'. Event add methods must return System.Runtime.InteropServices.WindowsRuntime.EventRegistrationToken. ...

Event add method '{0}' has return type '{1}'.  Event add methods must return System.Runtime.InteropServices.WindowsRuntime.EventRegistrationToken.  Was the module that you are exporting compiled as a winmdmodule (/target:winmdobj) for Windows Metadata export?