metafieldsSet mutation doesn't work

[ladygagaradio]

metafieldsSet mutation doesn't work

metafieldsset docs 

mutation metafieldsSet($metafields: [MetafieldsSetInput!]!) {
  metafieldsSet(metafields: $metafields) {
    metafields {
      id
    }
    userErrors {
      code
      field
      message

    }
  }
}

Error: Cannot query field "metafieldsSet" on type "Mutation"

It's a mutation just from documentation. What am I doing wrong? Trying to create a new metafield for a product variant

[odeyan]

Yes!
I have a same problem. 
Guys could you help please?

[Gift-o-the-Jab]

 It's the new supported types - needs json instead of json_string.

[odeyan]

Hey man! Thank you for your fastest answer! Can you explain please what you mean exactly? It will be awesome if you show the mutation example. Thanks a lot!