Shane Parkinson commented on How to set the default product variant through API?

It also doesn't work setting the option1 as null like.

{  
   "product":{  
      "body_html":"Every element of IntelliMouse Pro - from its unique arched shape to the texture of the rubber grip around its base - is the product of extensive customer and ergonomic research. Microsoft's popular wheel control, which now allows zooming and universal scrolling functions, gives IntelliMouse Pro outstanding comfort and efficiency.",
      "created_at":"2014-04-28T11:19:43",
      "published_at":"2014-04-28T11:19:43",
      "product_type":"Mice",
      "title":"Microsoft IntelliMouse Pro",
      "vendor":"Warner",
      "published":true,
      "sku":"MSIMPRO",
      "variants":[  
         {  
            "option1":null,
            "option2":null,
            "option3":null,
            "sku":"MSIMPRO",
            "barcode":null,
            "price":"49.99",
            "compare_at_price":null,
            "position":1,
            "inventory_management":"shopify",
            "inventory_policy":"deny",
            "fulfillment_service":"manual",
            "requires_shipping":true,
            "taxable":false,
            "inventory_quantity":null,
            "created_at":null,
            "grams":null
         }
      ]
   }
}