The purpose of the present study is to investigate whether or not the notional number attraction phenomenon is evident during L2 learners’ online processing of subject–verb agreement. L2 sentence processing studies have frequently reported the L2 learners’ online insensitivity to number mismatches, and it has been claimed that L2 learners have difficulty in representing grammatical number features, or they have a tendency to fail to access the number features in online tasks. L2 learners’ sensitivity to “notional number”, however, has not been considered by researchers. Hence, this research conducted a self-paced reading study targeting highly proficient Japanese learners of English (N = 28). The participants read the three types of sentences; (a) control (e.g., everyone in the room was/were…), (b) notional attraction (e.g., everyone in the team was/were…), and (c) grammatical attraction (e.g., everyone in the halls was/were…). In order to examine the participants’ sensitivity to number attractions, the differences in the reading times between the number matched and the mismatched verbs (was/were) were compared among the attraction types. The observed reading time differences in the grammatical and notional attraction condition were different than in the control condition, indicating that both attraction phenomena were evident. L2 learners’ imperfect representation of number features, which we call representation vulnerability, was discussed.