This can be caused by various things that are not allowed mathematically. For instance “divide by zero” or attempting to perform math on a text field.
You should use error trapping for things like “divide by zero”.
If you have a column defined as text you could consider changing it to a numeric type like int or long. Or you could explicitly convert it using int() (or whatever). But this would only work if it’s actually a number. You can’t convert a text value of “three” without writing (or finding) a custom function.
//