Unexpected error while processing request: undefined method `downcase' for 2:Fixnum

My Grape API is not working with grape v0.11.0 nor grape v0.12.0
Working with grape v0.10.0

I am using Rack Cascasde to load my API, ApiDoc with grape_swagger and my statics files
Using grape-activerecord v0.0.8, grape-entity 0.4.5 and grape-swagger 0.10.1

My database connection is loaded before invoking the API

It's only on the first GET that it fails
And it's difficult to see why and where in my project with this kind of trace !

Unexpected error while processing request: undefined method `downcase' for 2:Fixnum
    /var/lib/gems/2.1.0/gems/rack-1.6.4/lib/rack/utils.rb:501:in `[]'
    /var/lib/gems/2.1.0/gems/rack-1.6.4/lib/rack/response.rb:53:in `[]'
    /var/lib/gems/2.1.0/gems/activerecord-4.2.1/lib/active_record/connection_adapters/abstract/connection_pool.rb:650:in `call'
    /var/lib/gems/2.1.0/gems/grape-0.12.0/lib/grape/middleware/error.rb:27:in `block in call!'
    /var/lib/gems/2.1.0/gems/grape-0.12.0/lib/grape/middleware/error.rb:26:in `catch'
    /var/lib/gems/2.1.0/gems/grape-0.12.0/lib/grape/middleware/error.rb:26:in `call!'
    /var/lib/gems/2.1.0/gems/grape-0.12.0/lib/grape/middleware/base.rb:18:in `call'
    /var/lib/gems/2.1.0/gems/rack-1.6.4/lib/rack/head.rb:13:in `call'
    /var/lib/gems/2.1.0/gems/rack-1.6.4/lib/rack/builder.rb:153:in `call'
    /var/lib/gems/2.1.0/gems/grape-0.12.0/lib/grape/endpoint.rb:196:in `call!'
    /var/lib/gems/2.1.0/gems/grape-0.12.0/lib/grape/endpoint.rb:184:in `call'
    /var/lib/gems/2.1.0/gems/rack-mount-0.8.3/lib/rack/mount/route_set.rb:152:in `block in call'
    /var/lib/gems/2.1.0/gems/rack-mount-0.8.3/lib/rack/mount/code_generation.rb:96:in `block in recognize'
    /var/lib/gems/2.1.0/gems/rack-mount-0.8.3/lib/rack/mount/code_generation.rb:68:in `optimized_each'
    /var/lib/gems/2.1.0/gems/rack-mount-0.8.3/lib/rack/mount/code_generation.rb:95:in `recognize'
    /var/lib/gems/2.1.0/gems/rack-mount-0.8.3/lib/rack/mount/route_set.rb:141:in `call'
    /var/lib/gems/2.1.0/gems/grape-0.12.0/lib/grape/api.rb:98:in `call'
    /var/lib/gems/2.1.0/gems/grape-0.12.0/lib/grape/api.rb:33:in `call!'
    /var/lib/gems/2.1.0/gems/grape-0.12.0/lib/grape/api.rb:29:in `call'
    /var/lib/gems/2.1.0/gems/rack-1.6.4/lib/rack/cascade.rb:33:in `block in call'
    /var/lib/gems/2.1.0/gems/rack-1.6.4/lib/rack/cascade.rb:24:in `each'
    /var/lib/gems/2.1.0/gems/rack-1.6.4/lib/rack/cascade.rb:24:in `call'
    /var/lib/gems/2.1.0/gems/rack-contrib-1.3.0/lib/rack/contrib/post_body_content_type_parser.rb:36:in `call'
    /var/lib/gems/2.1.0/gems/rack-1.6.4/lib/rack/commonlogger.rb:33:in `call'
    /var/lib/gems/2.1.0/gems/sinatra-1.4.6/lib/sinatra/base.rb:218:in `call'
    /var/lib/gems/2.1.0/gems/rack-1.6.4/lib/rack/deflater.rb:35:in `call'
    /var/lib/gems/2.1.0/gems/thin-1.6.3/lib/thin/connection.rb:86:in `block in pre_process'
    /var/lib/gems/2.1.0/gems/thin-1.6.3/lib/thin/connection.rb:84:in `catch'
    /var/lib/gems/2.1.0/gems/thin-1.6.3/lib/thin/connection.rb:84:in `pre_process'
    /var/lib/gems/2.1.0/gems/eventmachine-1.0.7/lib/eventmachine.rb:1054:in `call'
    /var/lib/gems/2.1.0/gems/eventmachine-1.0.7/lib/eventmachine.rb:1054:in `block in spawn_threadpool'

Reproduce the first GET with Grape 0.11.0 or 0.12.0 and the listed Rack, ActiveRecord, and middleware versions. Start at rack/utils.rb:501 and rack/response.rb:53, then follow the Grape error path in grape/middleware/error.rb; done means the request no longer raises the reported Fixnum downcase error and its cause is documented.