Type to mock must be an interface or an abstract or non-sealed class

I'm using Moq to mock the following class in VB:

Public class Factory(Of T)
  Inherits Factory

  Public Overridable Shadows Readonly Property Model() as TModel
    Get
       ..
    End Get
  End Property

When I attempt to use Moq to mock the Model property in a unit test like so:

Dim factory As New Mock(of Factory(Of TestModel))
factory.Setup(Function(i) i.Model).Returns(new TestModel With { .. })

This fails with the error:

System.ArgumentException: Type to mock must be an interface or an abstract or non-sealed class. ---> System.TypeLoadException: Access is denied: 'PA.PSP.PFAD.Mvc.PFADFactory`1[PA.PSP.PFAD.Mvc.OrderExtensionsTest+TestModel]'. Result StackTrace: at System.Reflection.Emit.TypeBuilder.TermCreateClass(RuntimeModule module, Int32 tk, ObjectHandleOnStack type) at System.Reflection.Emit.TypeBuilder.CreateTypeNoLock() at System.Reflection.Emit.TypeBuilder.CreateType() at Castle.DynamicProxy.Generators.Emitters.AbstractTypeEmitter.CreateType(TypeBuilder type) at Castle.DynamicProxy.Generators.Emitters.AbstractTypeEmitter.BuildType() at Castle.DynamicProxy.Generators.ClassProxyGenerator.GenerateType(String name, Type[] interfaces, INamingScope namingScope)

Any idea why because while it's not an interface or abstract, it's not sealed (doesn't define NotInheritable in VB)....