Refactoring -"Make implicit" refactoring produces a compiler error on variables that are upcast to superclasses

Consider the following code:

public class Node
{
    public string Name { get; private set; }
    public Node Parent { get; private set; }
}

public class RootNode : Node
{
}

public class Tree
{
    private RootNode root;

    private Node FindNode(string name)
    {
        Node node = root;
        while (node != null)
        {
            if (node.Name == name)
            {
                return node;
            }

            node = node.Parent;
        }

        return null;
    }
}

The 'Implicit variable can be used' code issue reports that the variable 'node' in FindNode() can be made implicit. However, applying Make Implicit here causes a compiler error because it results in the subclass 'RootNode' being used. This is a problem because node.Parent returns the superclass 'Node'.