I am using Drupal 6.12 and cck 6.x-2.2

I created a text cck with checktboxes and several allowed values.
"Maximum number of values users can enter for this field." is set to "Unlimited" however, if I choose more than 15 fields only the first 15 are recorded, and I get the error message:
"An illegal choice has been detected. Please contact the site administrator."