Using MSVC 6

I have a class, and I'm relying on the "this" keyword quite a bit to perform some operations. There's a bug in the class, and I've been looking for it for quite a while - it compiles and everything, just doesn't work quite write. I think it's mostly fixed. But I decided to start up the debugger to see if that could help me at all, and in one of the MSVC debugging windows, I got the message
this CXX0017: Error: symbol "this" not found

I suspect that it is possible that this is the root of the error, but I have no idea how "this" could not be found... what is it telling me? Are there instances in which you can't use "this," or instances in which it fails?