title: "PHP Error: Call to a member function getKey() on string" from diff() method on Laravel Collection
body: <p>Here are the dumped collections:</p>
  
  <pre><code>//$collOne
  Illuminate\Database\Eloquent\Collection {#3386
    #items: array:6 [
      4807 =&gt; "{"color_id":7,"size_id":4,"pack_id":null}"
      4808 =&gt; "{"color_id":7,"size_id":2,"pack_id":null}"
      4809 =&gt; "{"color_id":7,"size_id":6,"pack_id":null}"
      4840 =&gt; "{"color_id":44,"size_id":4,"pack_id":null}"
      4841 =&gt; "{"color_id":44,"size_id":6,"pack_id":null}"
      4842 =&gt; "{"color_id":44,"size_id":2,"pack_id":null}"
    ]
  }
  //$collTwo
  Illuminate\Database\Eloquent\Collection {#3403
    #items: array:5 [
      430 =&gt; "{"color_id":7,"size_id":4,"pack_id":null}"
      431 =&gt; "{"color_id":7,"size_id":2,"pack_id":null}"
      433 =&gt; "{"color_id":44,"size_id":4,"pack_id":null}"
      434 =&gt; "{"color_id":44,"size_id":6,"pack_id":null}"
      435 =&gt; "{"color_id":44,"size_id":2,"pack_id":null}"
    ]
  }
  </code></pre>
  
  <p>Here is the error:</p>
  
  <pre><code>{
      "message": "Call to a member function getKey() on string",
      "exception": "Symfony\\Component\\Debug\\Exception\\FatalThrowableError",
      "file": "[obfuscated]/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Collection.php",
      "line": 281,
      "trace": [
          {
              "file": "[obfuscated].php",
              "line": 174,
              "function": "diff",
              "class": "Illuminate\\Database\\Eloquent\\Collection",
              "type": "-&gt;"
          },
  </code></pre>
  
  <p>Here is the line 174 referenced in the above error:</p>
  
  <pre><code>$diff = $collOne-&gt;diff($collTwo);
  </code></pre>
  
  <p>The docs seem pretty straight forward:</p>
  
  <blockquote>
    <p>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:</p>
  </blockquote>
  
  <p>Am I missing something obvious here?</p>