Why am I getting errors when posting Length in the product metafield?

Technical Q&A
1

We are trying to post Length in the metafield but we keep getting errors. We are making a call to

/admin/products/#{id}/metafields.json

and our body of post request contains the following:

{
 "metafield": { 
      "namespace": "global", 
      "key": "length", 
      "value": 5, 
      "value_type": "string"
   } 
}

We know the authentication is working because if we do the get request we can see the metafields data.
Any suggestion on what might be causing the issue?