Bad model query when number entered in text field

Let's see if I can explain.

We have a product with two attributes. One is a select list. The other is a text field.

The product SKU changes based on the value of the select list. However, if a number is put in the text field, the model adjustment lookup fails.

The problem can be traced to this function:

function uc_attribute_add_to_cart_data($form_values) {
  $combination = array();
  if (!isset($form_values['attributes'])) {
    return array('attributes' => array(), 'model' => null);
  }
  foreach ($form_values['attributes'] as $aid => $value) {
    if (is_numeric($value)) {
      $combination[$aid] = $value; // <-- Our text field is given numeric values, and added to this array.
    }
  }
  $result = db_query("SELECT model FROM {uc_product_adjustments} WHERE nid = %d AND combination LIKE '%s'", $form_values['nid'], serialize($combination));
  $model = db_result($result);
  // drupal_set_message('Attributes to add to order Array(aid => oid):<pre>'. print_r($form_values['attributes'], true) . $model .'</pre>');
  // Preserve the 'attributes' key to allow other modules to add to the data field.
  return array('attributes' => $form_values['attributes'], 'model' => $model);
}

So the query ends up as:

SELECT model FROM uc_product_adjustments WHERE nid = 425 AND combination LIKE 'a:2:{i:5;s:6:\"123123\";i:68;s:3:\"125\";}'

When it should be:

SELECT model FROM uc_product_adjustments WHERE nid = 425 AND combination LIKE 'a:1:{i:68;s:3:\"125\";}'

I don't see an obvious way to patch this, since hook_add_to_cart_data() only gives me the $form_values, when I really need to check the $form element to see if this is a text string (or, really, a free-form entry field).

Make sense?