Collection each iterator not modifying items

I’m trying to add a ‘modelType’ property to each item in a collection. However, it doesnt seem to be showing up afterward:

$results->each(function($row, $index) use ($modelType) {
$row[‘modelType’] = $modelType;

debug($results->toArray()); //<---- modelType isn’t listed in any of my items

I’ve figured out the issue.
The $results Collection was from a Query object, which had hydrate set to false. This of course produced arrays rather than entity objects.

Removing the hydrate(false) solved the issue.