iRule to replace { - } with %7B - %7D

I need to parse incoming headers and selectively replace { or } with %7B or %7D.

(v11.6.3)

Something like ?

when HTTP_REQUEST {

  if { [HTTP::uri] starts_with "/someuri/" } {

    HTTP::uri [string map {"/{/" "/%7B/"} [HTTP::uri]]

  }

}

But I suspect I'm not escaping the { character correctly.

01070151:3: Rule [/Common/test] error: /Common/test:3: error: [parse error: missing close-bracket][[string map {"/{/" "/%7B/"} [HTTP::uri]]

}]

TIA