FromJson for String

Stream: mathlib4

[Paige Thomas]

Do I need to derive this instance manually? If so, how would I do so?

structure VarName extends String
  deriving Inhabited, DecidableEq, Lean.FromJson

failed to synthesize instance
  Lean.FromJson (List Char)

[Paige Thomas]

instance : Lean.FromJson VarName :=
  { fromJson? := fun (json : Lean.Json) => do
    let str ← Lean.fromJson? json
    Except.ok (VarName.mk str) }

?