Surprising behavior with Generic deriving

[alexbiehl]

I am building out a Servant api containing the following type:

data NotVerified = NotVerified
  deriving stock (Generic)
  deriving anyclass (ToJSON, FromJSON)

data NotVerifiedResponse = NotVerifiedResponse { errorCode :: NotVerified }
  deriving stock (Generic)
  deriving anyclass (ToJSON, FromJSON)

Strangely, if I JSON encode the NotVerifiedResponse I get

encode $ NotVerifiedResponse { errorCode = NotVerified }
"{\"errorCode\":[]}"

I'd expect to see

"{\"errorCode\":\"NotVerified\"}"

I am using GHC 9.10.3 and aeson 2.2.3.0

[Lysxia]

By default, tagSingleConstructors is False so a type with a single nullary constructor is interpreted as a product, encoded as an array.