problem disabling input text field

I have an input text box. When the the number of characters entered in this text box reach a certain limit, i want to disable the text box.
my code -

import flash.text.TextField;
if(charCount == maxCharacters)
{
message_txt.enabled =false;
}

but i am getting the error
1119: Access of possibly undefined property enabled through a reference with static type flash.text:TextField.
how can i disable the text box?

Input text fields cannot be disabled. You must use a textfield component to change the editable propery. Either that or in actionscript create a TextField instance and change its 'type' property from input to dynamic.
If you want, you can always set the 'maxChars' property to set the maximum number of characters in the field, if that is what you were trying to do..

Hope that helps,
Mavrisa