Can't create items with REST API

[pitbuster]

I am trying to create items using the API, but I get this error

{
    "@context": "/api/contexts/Error", 
    "@type": "hydra:Error",
     "hydra:title": "An error occurred",
     "hydra:description": "The type of the \"quantity\" attribute must be \"int\", \"object\" given."
}

The same happens even if I use the Try it out forms in the /api/ page.

[pitbuster]

This was trying to set the quantity, but for my use case I can omit it and it works fine. I can close this if needed.