Around the age of 2, children freely drop subjects, irrespective of whether or not the target language is a null subject language. L. Haegeman noticed that the root character of subject drop suggests a topic-drop-type analysis, involving a discourse-bound null operator in the matrix SPEC of C binding a variable in subject position. The null element never ranges over a nonsingleton set; rather, it has its reference fixed to that of the antecedent. This interpretative difference correlates with a sensitivity to the weak crossover effect. L&S's proposal directly refers to cases of sentence-bound null operator constructions but can be immediately extended to the empty elements bound by discourse-identified null operators. If the language does not have the discourse-identified null operator the null constant is not permissible in SPEC-AGR configuration. A null element can be discourse-identified only if it is not c-commanded sentence-internally by a potential identifier.