C# - Improving encapsulation of property in this example?

I know of the error "The accessibility modifier of the set accessor must be more restrictive than the property or indexer". I also know the solution. Just not in this very specific case.

Consider this example:

    internal virtual bool IsFocused
    {
        get
        {
            return isFocused;
        }
        protected set
        {
            isFocused = value;
        }
    }
    private bool isFocused;

It shows the error. I just don't know why. How is "protected" not less accessible than internal? What would be the solution to this problem? I tried putting "internal protected" instead, without luck.