This seems to be related to #427

Currently using version 4.2.1

I run into the same issue when trying to run the assertion on nullable types, specifically I ran into it for int? and decimal?.

int? value = null;
value.ShouldBeAssignableTo<int?>();

decimal? value = null;
value.ShouldBeAssignableTo<decimal?>();

Both of those situations result in this message for their respective type:

Message: Shouldly.ShouldAssertException : value
    should be assignable to
System.Nullable`1[System.Decimal]
    but was
null