An undefined arithmetic operation (e.g., 1/0 or @LOG( -1)) occurred while LINGO was solving the model. If you have specified a row name for the constraint, LINGO will print the name of the constraint. If you haven't specified row names in your model, you may want to add them to assist in tracking down this error. Check the referenced constraint for any undefined operations and try to either, 1) rewrite it avoiding operations that can potentially become undefined, or, 2) use the @BND function to restrict variables to ranges where all operations are defined.