Getting list of Inherited types doesn't work for all types

I wrote a small method to list the inherited types but it doesn't works with a TreeNode for example:

Assume this classes:

class B { }
class A : B { }
class C :TreeNode { }

And then:

GetInheritedTypes(typeof(A)); //typeof(B)
GetInheritedTypes(typeof(C)); // 0 items

The method to list them:

List<Type> GetInheritedTypes(Type baseType) 
{ 
    return Assembly.GetAssembly(baseType)
                   .GetTypes()
                   .Where(type => type != baseType && type.IsAssignableFrom(baseType))
                   .ToList(); 
} 

Why is GetInheritedTypes(typeof(C)) returning 0 items instead of Typeof(TreeNode)?