CHARMM Development Project Forums User Discussion & Questions Parameter Set Discussion Patch for N-sultated Glucosamine - part 2

Concerning the impropers, you should never define them on nonplanar/tetrahedral centers (and not even on all planar centers; see this FAQ). So just remove the 3 superfluous IMPR lines from your topology file and you're done (I'm assuming the sulfamate nitrogen is not planar).