Session destroyed when POST

Question

I have a POST method in Sinatra:

post '/api/:token/boards/:identifier' do
    user = session[params[:token]]
    {:user=>user}.to_json
end'

If a do POST to this URI I receive null, and my userAccount is logged-out. If I try the same method with GET I receive my username.

get '/api/:token/boards/:identifier' do
    user = session[params[:token]]
    {:user=>user}.to_json
end'

Where is the problem?

Note: I have session enabled in config.ru

enable :sessions
set :session_secret, "super_secret"