It has been a long time, but I think it had something to do with a formula behind a report field. I think if the field was numeric it would crash if the value evaluated to null or empty string or something like that. I think I fixed it by forcing part of the expression to be an empty string or a space. This was definitely a bug and I had to find all the places in my complex reports where strings might be evaluating to numbers.
Even though I have since given up ever trying to use CR, I'd be surprised if this were not fixed by now, are you using a version that is this old?