This research addresses the question of why some children are disposed to making a large number of pronoun case errors and others are not. The answer proposed is that when pronoun paradigm building outstrips the development of INFL, children become especially vulnerable to erring in the choice of pronominal word form, resulting in pronoun case error. On the other hand, when pronoun paradigm building proceeds more conservatively, the risk of error is reduced. The spontaneous sentence production of children observed in naturalistic caregiver--child interaction from a cross-section of 44 children (2;0-4;0) is used to support this proposal. The data show that pronoun case error was minimal among children who had strong INFL. However, among children with weak INFL there was a wide range of variation, some children making many errors and others making none. Analysis of variance confirmed that this variation was strongly related to the dispersion of production attempts across an extended pronoun paradigm, such that, the fewer cells attempted, the lower the error rate. These findings show that pronoun case errors are not an inevitable result of grammatical development, but may conceivably be avoided altogether if paradigm building proceeds at a rate commensurate with the child's development of INFL.