Unexpected nimrod idetools behaviour on $data.str_val

idetools.nim

type
  Tparam_kind* = enum PK_EMPTY, PK_STRING

  Tparsed_parameter* = object
    case kind*: Tparam_kind
    of PK_EMPTY: nil
    of PK_STRING: str_val*: string

proc `$`*(data: Tparsed_parameter): string {.procvar.} =
  case data.kind:
  of PK_STRING: result = "\"" & $data.str_val & "\""
  else: nil

when isMainModule:
  var a : Tparsed_parameter
  a.kind = PK_STRING
  a.str_val = "hey"
  echo ($a)