LINGO will print this message for one of two reasons: 1) a conditional expression used to qualify a set operator function cannot be evaluated, or, 2) a subscript expression cannot be evaluated. When we say an expression cannot be evaluated, we mean that one or more variables in the expression are not fixed. LINGO will report the expression number, or row name (if used), where the fault occurred. Please be sure to add constraints or data statements to fix all the variables in the conditional or subscript expression in question. Also, if you are using primitive set member names in the model's equations you must use the @INDEX function to get the index of the primitive set member. If you don't use the @INDEX function, LINGO will treat the primitive set member as if it is a new scalar variable. You can have LINGO check for primitive set names that are in use in the both the data and the model equations by checking the Check for duplicate names in data and model box on the General Solver tab of the Solver|Options command's dialog box (or the SET CHKDUP 1 command from the LINGO command line). Finally, if you would like to use primitive set names directly in your model's equations (a practice we don't recommend), you can force LINGO to allow this by checking the Allow unrestricted use of primitive set member names checkbox on the General Solver tab (or the SET USEPNM 1 command from the LINGO command line).