C# Code First - How to create unique together foreign keys

Question

I have a model that has two foreign keys (as well as a primary key). I want to set the two fks unique together (meaning there can't be two columns with both having the same two fk values).

I've tried everything but I keep getting: "The type 'Model.MyType' must be a non-nullable value type in order to use it as a parameter 'T'". This error is in the Property function:

Property (o => o.MyType).HasColumnAnnotation (IndexAnnotation.AnnotationName,
new IndexAnnotation (new IndexAttribute ("UX_MyConstraint", 1) {IsUnique = true}));

(And another one with 2).

I prefer a fluent api solution but data annotations are fine too.

(C# entity framework 6.1 code first)

Thanks!