Japanese language learners aim to acquire reading, listening, writing and speaking skills. We at the Hinoki project (https://hinoki-project.org/) have recently been working on the Natsume collocation search system (https://hinoki-project.org/natsume/), the Natane learner corpus to support Natsume (https://hinoki-project.org/natane/) and the Nutmeg writing support system (http://hinoki-project.org/nutmeg/). In order to test the effectiveness of Nutmeg, we conducted an online experiment with 36 participants who used the system's register misuse identification feature to correct four writing assignments. Results show that Nutmeg can be an effective tool in correcting common register-related errors, especially those involving auxiliary verbs. However, the accuracy of verb and adverb identification was too low, suggesting the need for improvements in the variety of corpora used for identifying register misuse.