One of the most influential recent accounts of pronoun case-marking errors in young children's speech is Schutze & Wexler's (1996) Agreement/Tense Omission Model (ATOM). The ATOM predicts that the rate of agreeing verbs with non-nominative subjects will be so low that such errors can be reasonably disregarded as noise in the data. The present study tests this prediction on data from 12 children between the ages of 1;8.22 and 3;0.10. This is done, first, by identifying children who produced a reasonably large number of non-nominative 3psg subjects; second, by estimating the expected rate of agreeing verbs with masculine and feminine non-nominative subjects in these children's speech; and, third, by examining the actual rate at which agreeing verb forms occurred with non-nominative subjects in those areas of the data in which the expected error rate was significantly greater than 10%. The results show, first, that only three of the children produced enough non-nominative subjects to allow a reasonable test of the ATOM to be made; second, that for all three of these children, the only area of the data in which the expected frequency of agreeing verbs with non-nominative subjects was significantly greater than 10% was their use of feminine case-marked subjects; and third, that for all three of these children, the rate of agreeing verbs with non-nominative feminine subjects was over 30%. These results raise serious doubts about the claim that children's use of non-nominative subjects can be explained in terms of AGR optionality, and suggest the need for a model of pronoun case-marking error that can explain why some children produce agreeing verb forms with non-nominative subjects as often as they do.