The type '%4!ls!' cannot be used as type parameter '%3!ls!' in the generic type or method '%1!ls!'. The nullable type '%4!ls!' ...

The type '%4!ls!' cannot be used as type parameter '%3!ls!' in the generic type or method '%1!ls!'. The nullable type '%4!ls!' does not satisfy the constraint of '%2!ls!'. Nullable types can not satisfy any interface constraints.