Update Well, cacharbe is probably right, so what I say may not be applicable. I'm glad that's never happened to me :) /update
You don't modify anything in the program do you -- I mean modify anything in Excel with the program? The negative integer (overflow?) sounded familiar, but I didn't find it in the docs...
(There was this, but I don't think it applies here)
Randomly failing method calls
It seems like modifying objects that are not selected/activated is som
+etimes fragile. Most of these problems go away if the chart/sheet/doc
+ument is selected or activated before being manipulated (just like an
+ interactive user would automatically do it).
If you're modifying any of the values within the program it by be noteworthy to see that code -- maybe something went wrong there? But (as I suspect since you say you saw it in Excel correctly so I guess you're not modifying anything), you may want to go to the Parse Excel module listed by someone else below.
The only thing that comes to mind is that you might be referring to a cell that is out of bounds -- but then Excel would proably raise an exception, so its probably not that.