XNA Guide text input - maximum length

so I am using Guide.BeginShowKeyboardInput to get the user to enter their username. I would like this to be limited to 20 characters, and it seems to break expected behaviour to let them input whatever they like and trim it later - so how would I go about limiting what they can input in the text box itself? I have the following code:

public string GetKeyboardInput(string title, string description, string defaultText, int maxLength)
    {
        if (input.CheckCancel())
        {
            useKeyboardResult = false;
            KeyboardResult = null;
        }

        if (KeyboardResult == null && !Guide.IsVisible)
        {
            KeyboardResult = Guide.BeginShowKeyboardInput(PlayerIndex.One, title,
                                                          description, defaultText, null, null);
            useKeyboardResult = true;
        }
        else if (KeyboardResult != null && KeyboardResult.IsCompleted)
        {
            string result = Guide.EndShowKeyboardInput(KeyboardResult);
            KeyboardResult = null;
            if (result == null)
            {
                useKeyboardResult = false;
                return null;
            }

            if (useKeyboardResult)
            {
                KeyboardResult = null;
                return result;
            }
        }
        else //the user is still entering inputs
        {

        }

        return null;
    } 

I assume the code I need would go in that final, empty else{} block, but I can't see any way to do this. Does anyone know how?