diff() method causes `PHP Error: Call to a member function getKey() on string` on Laravel Collection

Here are the dumped collections:

//$collOne
Illuminate\Database\Eloquent\Collection {#3386
  #items: array:6 [
    4807 => "{"color_id":7,"size_id":4,"pack_id":null}"
    4808 => "{"color_id":7,"size_id":2,"pack_id":null}"
    4809 => "{"color_id":7,"size_id":6,"pack_id":null}"
    4840 => "{"color_id":44,"size_id":4,"pack_id":null}"
    4841 => "{"color_id":44,"size_id":6,"pack_id":null}"
    4842 => "{"color_id":44,"size_id":2,"pack_id":null}"
  ]
}
//$collTwo
Illuminate\Database\Eloquent\Collection {#3403
  #items: array:5 [
    430 => "{"color_id":7,"size_id":4,"pack_id":null}"
    431 => "{"color_id":7,"size_id":2,"pack_id":null}"
    433 => "{"color_id":44,"size_id":4,"pack_id":null}"
    434 => "{"color_id":44,"size_id":6,"pack_id":null}"
    435 => "{"color_id":44,"size_id":2,"pack_id":null}"
  ]
}

Here is the code causing the error:

$diff = $collOne->diff($collTwo);

The docs seem pretty straight forward:

The diff method compares the collection against another collection or a plain PHP array based on its values. This method will return the values in the original collection that are not present in the given collection:

Am I missing something obvious here?

via Chebli Mohamed