Why can't I use an array of term slugs in WP_Query?

Question

In WP_Query, a comma-separated list of term slugs works fine:

$query = new WP_Query ( array(  
     'customtaxonomy' => 'term1,term2'
) );

...but an array of slugs causes a warning:

$query = new WP_Query ( array(  
     'customtaxonomy' => array( 'term1', 'term2' )
) );

Warning: urlencode() expects parameter 1 to be string, array given in /path/to/wp-includes/formatting.php on line 3253

Another WPSE post proposes that this could be WP looking for hierarchy in a hierarchical taxonomy, but I can't think of any way to make sense of this behaviour. Do you know why this is happening?

Answer 1

If you read the codex page carefully, you will see that taxonomy parameter {tax} (in your case it is customtaxonomy) takes only string value. Furthermore such taxonomy parameter is deprecated from version 3.1. So in your case you have to pass tax_query array with terms array:

$query = new WP_Query ( array(  
    'tax_query' => array( 
        'terms' => array( 'term1', 'term2' )
    )
) );