To teach adaptively, teachers should be able to take the students' level of knowledge into account. Therefore, a key component of pedagogical content knowledge (PCK) is the ability to assume the students' perspectives. However, due to the so-called expert blind spot, teachers tend to misestimate their students' knowledge, such as when estimating the difficulty of a given task. This empirically well-documented estimation bias is predicted by Nickerson's anchoring and adjustment model, which generally explains how people take on other people's perspectives. In this article, we present an intervention study that aims to improve the accuracy of prospective teachers' judgments of task difficulty in the area of functional thinking. Two types of treatments are derived from Nickerson's model. In the first condition (PCK group), participants acquire knowledge about task characteristics and students' misconceptions. The second condition (sensitizing group) serves to control the idea that potential improvements in the PCK group are not merely based on a pure sensitization of the expert's estimation bias. Accordingly, these participants are only informed about the general tendency of overestimating task difficulties. The results showed that the PCK group improved both in terms of the accuracy of the estimated solution rates and the accuracy of rank order, whereas the sensitizing group only improved in regard to the former. Thus, the study shows that prospective teachers' diagnostic judgments can be improved by teaching them relevant PCK in a short amount of time.