Error: Unable to serialize object as a key

[sproctor]

Running gatsby build fails on https://github.com/gatsbyjs/gatsby-starter-sanity-homepage-ts

Error message:

 ERROR #85901  GRAPHQL.UNKNOWN

There was an error in your GraphQL query:

Unable to serialize object as a key:
{"_ref":"-27fd89a4-d3ef-5ebe-b5c8-538e2fe5e7b8","_type":"reference"}

  16 |             href
  17 |             text
  18 |             description
  19 |             icon {
  20 |               alt
  21 |               gatsbyImageData
  22 |             }
  23 |           }
  24 |         }
  25 |       }
> 26 |       cta {
     |       ^
  27 |         id
  28 |         href
  29 |         text
  30 |       }
  31 |     }
  32 |   }
  33 | }

This problem started with 7.7.0 and persists in 7.9.1. 7.6.3 works fine.