The present paper is devoted to the numerical study of an extensional rheometer where a fluid sample is stretched between two plates. We use the FENE-CR model in order to simulate a Boger fluid stretched in the device. The analysis shows that the flow is not purely extensional, and that experimental data have to be analysed with caution, in particular if the extensional viscosity is not very high. In general, numerical simulations allow us to better understand experimental data.