Using Mocks for Design Data in WPF

Question

I have a WPF module that gets data in form of interfaces, e.g. ICustomer. The real Customer object is defined in another module, that I can't reference in the WPF module because of lose coupling.

Since we want design data in our projects, and I want to avoid to write Fakes for every entity, I thought that using Moq would help here, so I made a small example:

public MainVM()
{
    if (IsInDesignMode)
    {
        var c1 = new Mock<ICustomer>();
        c1.SetupAllProperties();
        c1.FirstName = "John";
        c1.LastName = "Doe";

        Customers = new List<ICustomer>
        {
            c1.Object,
        };
    }
}

In the view I bring in the datacontext and do:

d:DataContext="{d:DesignInstance vm:MainVM, IsDesignTimeCreatable=True}"

The problem now is, that the line in the view throws me an exception:

Unable to cast object of type 'Castle.Proxies.ICustomerProxy_5' to 'Prototype.Data.ICustomer'

How can I get rid of this error? Is there some workaround?