Fix exception message

birdvisu/providers.py

@@ -191,7 +191,7 @@ class OspfFileTopologyParser:
 			# it must match.)
 			dr_id, discr = tgt.lstrip(r'[').rstrip(']').split('-')
 			candidates = list(filter(lambda vid: vid.dr_id == dr_id and vid.discriminator == discr, result.vertices))
-			if len(candidates) != 1: raise OspfFileParseError(f'Multiple candidates for uniquely determined network {tgt}')
+			if len(candidates) != 1: raise OspfFileParseError(f'Multiple or no candidates for uniquely determined network {tgt}')
 			tgtid = candidates[0]
 			edge = Edge(source=src, target=tgtid, cost=cost, count=count)
 			result.add_edge(edge)