Purescript: why can't genericEncodeJSON encode types inside a container?

I don't understand why the below program fails to compile.

module Main where

import Prelude
import Effect (Effect)
import Effect.Console (log)
import Data.Generic.Rep (class Generic)
import Foreign.Generic (defaultOptions, genericEncodeJSON)

main :: Effect Unit
main = do
  log "Hello sailor!"

newtype Foo = Foo {
    x :: Int
  , y :: Int
  , z :: Array Bar
}

type Bar = {
    a :: String
  , b :: String
}

derive instance repGenericFoo:: Generic Foo _

fooToJSON :: Foo -> String
fooToJSON = genericEncodeJSON $ defaultOptions { unwrapSingleConstructors = true }

This is the output by the compiler

Compiling Main
Error found:
in module Main
at src/Main.purs line 27, column 13 - line 27, column 30

  No type class instance was found for

    Foreign.Class.Encode { a :: String
                         , b :: String
                        }

If Array is not used, then the program compiles

newtype Foo = Foo {
    x :: Int
  , y :: Int
  , z :: Bar
}

type Bar = {
    a :: String
  , b :: String
}

why is it that genericEncodeJSON can't encode Bar if it's inside a container like Array or List?