BizTalk Unit Tests

I am trying to execute Unit Tests against a schema that uses imports.  All of my unit tests against the sub-schema work, but when it runs against the main schema, it fails.  The error message is as follows

TOMTreeGenerator.GetElementAbsoluteType: Object reference not set to an instance of an object.

I assume, it is unable to find the schema that the import points to.  Can anyone point me in the right direction?

Thanks,

Todd