PathPrefixStrip just doesn't

Wanting to proxy localhost:81/es to localhost:9200/ but having no joy - the /es is just not stripped.

Config

[frontends]
  [frontends.ES]
  entrypoints = ["http"]
  backend = "ES"
    rule = "Path:/es;PathPrefixStrip:/es"

[backends]
  [backends.ES]
    [backends.ES.servers.server1]
      url = "http://127.0.0.1:9200"

Request

http://localhost:81/es/_cat

Response

{
    error: "Incorrect HTTP method for uri [/es/_cat] and method [GET], allowed: [POST]",
    status: 405
}

This is the exact same response as calling localhost:9200/es/_cat so no stripping here.