Tuition calculation, using equation on class fee

Hello community, I am using equation on class fee and when I am going to calculate the tuition I am getting this message, could someone help me?. When I tested it worked, but I dont know why it doesn't take the value from the student.

Uninitialized global in equation %1 near line %2 for %3. (14460,322)

The specified global variable had not been initialized prior to reference.  You must either establish its value via an ASSIGN statement or pass the value in when you invoke the equation processor interpreter.

regards, Octani.