Check the PDB file to ensure that the ring atoms are properly named. For most monosaccharides, they should be C1, C2, C3, C4, C5. If the residue is a sialic acid, the ring atoms should be C2, C3, C4, C5, C6, and O6.
Example error message from command line:
ERROR) vecsub: two vectors don't have the same size