title: Bigcommerce cart 422: Missing required fields error
body: <p>I'm using Laravel to consume the Bigcommerce V3 API.</p>
  <p>I have ben able to succesfully create a new cart. But when trying to add an item to it, I keep getting a <code>422: Missing required fields</code> error.</p>
  <p>I'm making my request trough Guzzle like this:</p>
  <pre><code>return json_decode($this-&gt;client-&gt;getRestClient()
              -&gt;post('carts/'.$cartId.'/items?include=line_items.physical_items.options', [
                  'Accept' =&gt; 'application/json',
                  'json' =&gt; [
                      'line_items' =&gt; [
                          'product_id' =&gt; 86,
                          'quantity' =&gt; 1
                      ],
                  ],
              ])
              -&gt;getBody())
          -&gt;data;
  </code></pre>
  <p>The product I'm trying to add has no options or modifiers, so I don't understand what could wrong with my request. According to the <a href="https://developer.bigcommerce.com/api-reference/store-management/carts/cart-items/addcartlineitem" rel="nofollow noreferrer">docs</a>, this should be all that's needed.</p>
  <p>Does anyone have any idea what could be wrong? I tried contacting support, but to no avail.
  Thanks in advance!</p>