One of the differences between Chinese and English is that the former allows both null subjects in finite sentences and null objects, but the latter allows neither. This cross-linguistic variation is believed to be related to the underspecification of I and topic drop in Chinese but not in English. This paper reports on an empirical study investigating the unlearning of null subjects and null objects by 159 Chinese learners in their L2 acquisition of English. In L1 acquisition, it has been found that English-speaking children display an asymmetry by frequently allowing null subjects but rarely null objects. The results of this study indicate that there is an asymmetry in Chinese learners' L2 English, which, however, is opposite to that found in English L1 acquisition: Chinese learners are able to reject the incorrect null subject in English, but unable to detect the ungrammaticality of the null object. It is proposed that the unlearning of null subjects by Chinese learners of English is triggered by the evidence in their input indicating the specification of AGR(eement) and T(ense) in English, and that the difficulty in the unlearning of null objects is related to the lack of informative evidence to unset the [+ topic-drop] setting in Chinese learners' L2 English.