Why is Core Data Model URL (modelURL) returning as nil?

When I create a new project with CoreDataModel created by XCode, all is well. However, I added a core model file to my project and the "core data stack" code. The code is properly getting invoked, however, the model's URL is nil. I have tried to examine the project and cannot determine if the model file is not properly added, or compiled, or if the URL just cannot be found.

Help me debug and resolve.

NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"Model" withExtension:@"momd"];

modelUrl returns as 'nil.'

via Chebli Mohamed