path helper generating exeception on console, but generating path for new record in webpage

I am trying to modify my script but i am facing a strange issue. I see that when i use a path helper like this

helper.link_to customer_email, app.store_customer_path(store,order.customer_id) , when order.customer_id is blank i get a exception on console as below

ActionController::RoutingError: store_customer_url failed to generate from {:controller=>"customers", :action=>"show", :store_id=#<object>}

. but when i load a web page i see that i get a path generated to create a new customer.

I am not able to understand this and its confusing me, can anyone please help, Thanks.