In this study, we investigated the debugging process that early childhood preservice teachers used during block-based programing. Its purpose was to provide insights into how to prepare early childhood teachers to integrate computer science into instruction. This study reports the types of errors that early childhood preservice teachers commonly made and how they debugged the errors. Findings are discussed in relation to research and practice that could benefit from debugging instruction. This study provides directions for future computer science education research that aims to prepare teachers for programming, computational thinking, and STEM education. Though this study used robotics as a programming context, findings on early childhood preservice teachers' debugging processes could be applicable to other contexts involving block-based programming.