In classrooms tests, students are regularly required to demonstrate their understanding of mathematical concepts. When children encounter problems in demonstrating such understanding, it is often not clear whether this is because of the language of the teachers' questions and instructions or a genuine non-understanding of the concept itself. This paper uses Conversation Analysis to investigate the role that language plays in Year 1 oral maths assessment in an Australian Indigenous community school. This approach allows us to monitor the very subtle communicative gestures, verbal and non-verbal, that contribute to the trajectory of a particular test task. Here we are able to bring to light a range of ways in which language may interfere with demonstrations of understanding of mathematical concepts. These include particular mathematical words (e.g., "size," "shape," "same"), as well as problems with what is being asked in an instruction. We argue that while all children must learn new mathematical language in their early years of schooling, the challenge for the students we have recorded may be compounded by the language differences between the Indigenous variety of language they speak in the community, and the Standard Australian English of the classroom and teachers.