How do I create a unique index on multiple JSONB fields in Prisma?

I can create a unique index on a Postgresql Json column with this:

@@unique([car])

But, I need to create a unique index based on the year, model and color:

@@unique([car->>'year', car->>'model', car->>'color'])

I instead get:

Error: Prisma schema validation - (get-config wasm)
Error code: P1012
error: Error validating: This line is not a valid field or attribute definition.
  -->  schema.prisma:25