makewav doesn't support multi-segment DECB binaries. It writes out non-gapped cassette files which doesn't allow non-contiguous data. It must either pad the open holes, or write gapped cassette files instead. The solution for makewav could be to first analyse the DECB file and then choose which way to go.
EDIT: For now I made it print a better error message if a multi-segment DECB is detected.