Background: Research has shown that mathematics anxiety negatively correlates with primary school mathematics performance, including fraction knowledge. However, recently no significant correlation was found between fraction arithmetic performance and state anxiety measured after the fraction task. One possible explanation is the natural number bias (NNB), a tendency to apply natural number reasoning in fraction tasks, even when this is inappropriate. Students with the NNB may not realize they are answering incorrectly. Aims: The aim is to examine whether a misconception, namely the NNB, can influence students' fraction state anxiety. Sample: The participants were 119 fifth- and sixth-grade students categorized as belonging to an NNB group (n = 60) or a No-NNB group (n = 59), according to their NNB-related answering profile on a fraction arithmetic task. Methods: Group differences were examined for state anxiety and performance on a fraction and a whole number arithmetic task and self-reported trait mathematics anxiety. Results: The NNB group reported lower fraction state anxiety than the No-NNB group, but there was no significant difference in trait mathematics anxiety. Furthermore, the NNB group reported lower fraction state anxiety than whole number state anxiety, while the opposite was true for the No-NNB group. Conclusion: The present study suggests that students' perceptions of their own performance influence their state anxiety responses, and students with a NNB may not be aware of their misconception and poor performance. Not taking into account qualitative differences in low performance, such as misconceptions, may lead to misinterpretations in state anxiety-performance relations.