I think only two lines could cause this
@echantillon\.set\_eros\_values
if @echantillon\.save
Which line does it say the error comes from in your output?
You could write some
Rails.logger.debug "Hi, this is the current status: #{@echantillon.inspect}"
between every line and then watch your logs/development.log to see where exactly it breaks.
I would guess on set_eros_values (whatever that function is supposed
to do 