ArgumentError in rack-mount route_set.rb

Backtrace:

ArgumentError
.../escdef/vendor/gems/dirs/rack-mount/lib/rack/mount/generation/route_set.rb:136:in `extract_params!'
.../escdef/vendor/gems/dirs/rack-mount/lib/rack/mount/generation/route_set.rb:36:in `url'
.../escdef/vendor/gems/dirs/rails/actionpack/lib/action_dispatch/routing/route_set.rb:387:in `generate'

ActionDispatch, routing/route_set.rb, line 387 passes 4 args to rack-mount's route_set.rb, which raises an argument error in extract_params!. The args appear to be a string and 3 hashes, so perhaps all thats needed is a merge of the hashes? Here's #pp of the args triggering the ArgError:

["escalation_definitions",
 {:controller=>"escalation_definitions"},
 {:action=>"index"},
 {:parameterize=>
   #<Proc:0x00007fe04ee30a68@/home/rando/development/api/escdef/vendor/gems/dirs/rails/actionpack/lib/action_dispatch/routing/route_set.rb:377>}]