Method Type.IsAssignableFrom(Type c) call NullReferenceException, when property UnderlyingSystemType = null

[andrey-kosteley]

Kind time of day.
Sorry for my English.

Environment:
I have a custom type - "MyType", the heir from the type "Type".
The "UnderlyingSystemType" property of my class is overridden and returns "null".
There is a variable of type "MyType", let's call it "myType_Variable".
And there is a variable of any type, let's call it "anyType_Variable"

A task:
Perform the method - Type.IsAssignableFrom(Type c)

I get:
NullReferenceException: Object reference not set to an instance of an object

Analysis:
I found out that the problem is in the IsRuntimeImplemented() method and the implementation of the IsAssignableFrom(Type c) method.

How do I think the problem is in the falsely flashing method IsAssignableFrom(Type c) from the .NET Framework.
Code from .NET Frameworks:
...
RuntimeType underlyingSystemType = this.UnderlyingSystemType as RuntimeType;
if (underlyingSystemType != null)
{
return underlyingSystemType.IsAssignableFrom(c);
}
...

I understand that I can override method IsAssignableFrom(Type c), I did so, but it seems to me that this use case should be out of the box. method IsAssignableFrom(Type c) is not mandatory for implementation.