How do i validate checkbox array

[eluser]123wesweat[/eluser]
i am trying to validate if minimal 1 check box is selected

Code:

<input type="checkbox" name="colors[]" value="red"/>
<input type="checkbox" name="colors[]" value="white"/>
etc

validation rule

Code:

$val->set_rules('colors', 'Color', 'trim|required|xxs_clean');

but i get this error

Code:

Message: Array to string conversion

Filename: libraries/Form_validation.php