Humans are believed to be equipped with an Approximate Number System (ANS) that supports non-symbolic representations of numerical magnitudes. Acuity in the ANS is thought to progress developmentally from childhood to adolescence and to be related to mathematical achievement. However, from previous research, it is not clear whether it is possible to train this core ability. The present study investigates the effect of substantive corrective feedback on non-symbolic number comparisons, in an adult population, using a control group comparison design. The results indicate that even extensive training with feedback (1000 trials) has no effect on ANS-acuity. However, feedback induces a motivational effect on participants. These results suggest that some of the observed relationship between ANS-acuity and general math achievement may be mediated by a motivational factor and that a characteristic of the number sense may be its non-plasticity, at least for adults.