Grab array / list of Contexts?

$c = $modx->newQuery('modContext');
$c->where(array(
   'key:!=' => 'web',
));
$contexts = $modx->getCollection('modContext',$c);

$c->select('modContext.key');

$c->select('key');

Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 3181236 bytes)

<?php
$c = $modx->newQuery('modContext');
$c->where(array(
   'key:!=' => 'web',
));
$contexts = $modx->getCollection('modContext',$c);

$contextNames = array();

foreach($contexts as $context) {
     $contextNames[] = $context->get('key');
}