Generic for newtype not found when in a Sum type

[tonicebrian]

I have these data structures:

import Data.Generic.Rep (class Generic)

opts :: Options
opts = defaultOptions { unwrapSingleConstructors = true }

data Sum
  = Nullary
  | Unary Int
  | UnaryNewType WrappedInt

newtype WrappedInt
  = WrappedInt Int

derive instance sumGeneric :: Generic Sum _

derive instance wrappedGeneric :: Generic WrappedInt _

If I do

genericEncodeJSON opts (WrappedInt 8)

I corretcly get a 8 but If I do

genericEncodeJSON opts (UnaryNewType (WrappedInt 8))

I get an error about not being able to find the generic instance that was used in the first example:

  No type class instance was found for                                                                                                      
                                           
    Foreign.Generic.Class.Encode WrappedInt
                                           

while applying a function genericEncodeJSON
  of type Generic t0 t1 => GenericEncode t1 => { fieldTransform :: String -> String 
                                               , sumEncoding :: SumEncoding         
                                               , unwrapSingleArguments :: Boolean   
                                               , unwrapSingleConstructors :: Boolean
                                               }                                    
                                               -> t0 -> String                      
  to argument opts
while inferring the type of genericEncodeJSON opts
in value declaration main

What am I doing wrong here?