Add querystring parameter without value

I need Kong to add a queryparamtert “wsdl” on requests matching a given route so that requests are transformed from “” to “”. However, I’m unable to add a querystring paramter without giving it a value.

I was expecting this config to might do the trick:
- name: request-transformer
route: foo.var
- wsdl

But this gives me this error:
/usr/local/share/lua/5.1/kong/cmd/config.lua:111: Failed parsing:
in ‘plugins’:

  • in entry 4 of ‘plugins’:
    in ‘config’:
    in ‘add’:
    in ‘querystring’:
    - in entry 1 of ‘querystring’: key ‘wsdl’ has no value

How can I achieve this using Kong?