to_json / of_yojson attribute example doesn't compile

[gs0510]

The compilation of the example here fails with

File "_none_", line 1:
Error: Unbound value StringMap.to_yojson

My workaround right now is to define:

type attrs = string StringMap.t
let attrs_to_json m = StringMap.bindings m |> [%to_yojson: (string * string) list]

[RCmerci]

same problem with ppx_deriving_yojson 3.5.1