Alt text doesn't appear to be accessible in GraphQL

[tkatchenko]

When I run *[_type == 'sanity.imageAsset']{alt} in the Sanity studio Vision tab I get a list of image assets and their alt text but when I run:

{
  allSanityImageAsset {
    alt
  }
}

in the deployed GraphQL playground, I get a "Cannot query field 'alt' on type 'SanityImageAsset'.".

Is this the correct structure for the getting the alt text? Not sure if I'm missing something as I can't seem to find it via GraphQL.