It may be connected in the model, but if you're using custom SKEYS, and/or Usershapes, make sure you have the correct amount of connection points. For example, if you build a custom usershape and assign the CAP skey, the usershape is required to have two connection points.

If there are not usershapes in the line, I'm not sure of what else could be the cause of Error 13, but I would be interested in hearing if you find a fix.