Inside of a protected method, how can you determine if it is being called inappropriately?

The Extra Credit for the Methods series in the Ruby Foundations course is this:

Now that we know the differences between the different kinds of methods, try writing a program with a protected method. Have that method raise a specific custom error if it's called inappropriately.

One thing I've tried is creating a custom error by inheriting from StandardError (or whatever) and then catching NoMethodError within a protected method and just raising my custom error. Like this:

class CustomError < StandardError
end

class MyClass

    protected
    def my_protected_method
        begin
            # ...
        rescue NoMethodError
            raise CustomError.new("Custom Error Message")
        end
    end
end

Unfortunately this doesn't work. I'm assuming because NoMethodError is thrown before any of the code inside the method gets executed. So, I think I can only catch a NoMethodError if I do so outside of the protected method like this:

begin
    class_instance.my_protected_method
rescue NoMethodError
    raise CustomError.new("Custom Error Message")
end

But of course, that does not fulfill the requirements of the assignment, which says:

...try writing a program with a protected method. Have that method raise a specific custom error if it's called inappropriately.

So, this way seems to be a dead end. How can I tell within a protected method if that method is being called inappropriately? The videos do not go over this, so I've had to look elsewhere.