Setting a default checkbox value in FAPI Trying to set a default value on an entity reference field.

$unitid = node_load($unit);
$form['field_choose_a_service']['und']['#default_value'] = $unitid;



  Warning: Illegal offset type in form_type_checkboxes_value() (line
  2390 of
  /homepages/43/d503288075/htdocs/example.co.uk/includes/form.inc).
  
  Notice: Object of class stdClass could not be converted to int in
  form_process_checkboxes() (line 3316 of
  /homepages/43/d503288075/htdocs/example.co.uk/includes/form.inc).


And targeting the nid from that loaded node...

$unitid = node_load($unit);
$form['field_choose_a_service']['und']['#default_value'] = $unitid-&gt;nid;



  Warning: Invalid argument supplied for foreach() in
  form_type_checkboxes_value() (line 2389 of
  /homepages/43/d503288075/htdocs/example.co.uk/includes/form.inc).


The first one actually works, but kicks up the errors. What am I doing wrong?

Essentially I want to check if the nid im passing in the URL as a _GET query matches an option in the entity reference checkbox options, and make that option selected if it does.
Try this one:

$unitid = node_load($unit);
$form['field_choose_a_service']['und']['#default_value'] = array($unitid-&gt;nid);


And please note that node_load returns an object but not a ID. I see you use $unitid as the variable name. It can make people confused.