This can happen if the staff lines are undetected. Most commonly, it's because the staff lines were tilted instead of being completely horizontal.
This is because the algorithm failed to locate the clefs that you didn't specify for. Most likely a treble clef that was skinnier than the templates I prepared the algorithm for will go undetected! (Certtainly happends with bass clefs as well, espcially small inline clefs) I am working on it :)
I wanna hear it! Ask a question at clefconverter@gmail.com, or create a Github issue here!