The Sentence Structure sub-test (SST) of the Clinical Evaluation of Language Fundamentals (CELF) aims to "measure the acquisition of grammatical (structural) rules at the sentence level". Although originally designed for clinical practice with monolingual children, components of the CELF, such as the SST, are often used to inform psycholinguistic research. Raw scores are also commonly used to estimate the English proficiency of bilingual children. This study queries the reliability of the SST as an index of children's ability to deal with structural complexity in sentence comprehension, and demonstrates that cognitive complexity induces a considerable confound in the task, affecting 5- to 7-year-old monolinguals (n = 87) and bilinguals (n = 87) alike.