What does "Argument #1 is not an array in _form_set_class()" mean? In Drupal 7, I'm getting this error and want to fix it:


  
  Warning: array_merge() [function.array-merge]: Argument #1 is not an array in _form_set_class() (line 4021 of {my site}/includes/form.inc).   
  Warning: array_merge() [function.array-merge]: Argument #1 is not an array in _form_set_class() (line 4021 of {my site}/includes/form.inc).   
  Warning: implode() [function.implode]: Invalid arguments passed in theme_radios() (line 2746 of {my site}/includes/form.inc).
  


What does this mean, and what should I do about it?
For me it happened; when I used 

$form['field']['#attributes'] = array('class' =&gt; 'myclass'); 

While it should be

$form['field']['#attributes'] = array('class' =&gt; array('myclass'));