All files must be coordinate-sorted error?

I am trying to use sambamba to an indexed whole genome dataset.

basename_X $dataset
f="$(basename_X -- $dataset)"
sambamba depth base -L genomic.bed $dataset> ./reads/"$f.txt"

However, the above script gives the following error.

sambamba-depth: All files must be coordinate-sorted

What is the reason and how can I solve it?