Determining the right code reviewer for a given code change requires understanding the characteristics of the changed code, identifying the skills of each potential reviewer (expertise profile), and finding a good match between the two. To facilitate this task, we design a code reviewer recommender that operates on the knowledge units (KUs) of a programming language. We define a KU as a cohesive set of key capabilities that are offered by one or more building blocks of a given programming language. We operationalize our KUs using certification exams for the Java programming language. We detect KUs from 10 actively maintained Java projects from GitHub, spanning 290K commits and 65K pull requests (PRs). We generate developer expertise profiles based on the detected KUs. We use these KU-based expertise profiles to build a code reviewer recommender (KUREC). We compare KUREC’s performance to that of seven baseline recommenders. KUREC ranked first along with the top-performing baseline recommender (RF) in a Scott-Knott ESD analysis of recommendation accuracy (the top-5 accuracy of KUREC is 0.84 (median) and the MAP@5 is 0.51 (median)). From a practical standpoint, we highlight that KUREC’s performance is more stable (lower interquartile range) than that of RF, thus making it more consistent and potentially more trustworthy. We also design three new recommenders by combining KUREC with our baseline recommenders. These new combined recommenders outperform both KUREC and the individual baselines. Finally, we evaluate how reasonable the recommendations from KUREC and the combined recommenders are when those deviate from the ground truth. We observe that KUREC is the recommender with the highest percentage of reasonable recommendations (63.4%). Overall we conclude that KUREC and one of the combined recommenders (e.g., AD_HYBRID) are overall superior to the baseline recommenders that we studied. Future work in the area should thus (i) consider KU-based recommenders as baselines and (ii) experiment with combined recommenders.