You should really mention if the function is
- Called from an Excel cell.
- Has an event handler.
- Show how your variables are declared.
If called from a cell, the inputs to the function can cause you problems with different inputs. Sometimes preventing the call of the function if the types significantly change to something unexpected. For example a parameter declared as variant in its signature will pass in an error but fail in the function. You may be trapping this error and returning #VALUE in the code. No way for us to know that.
If you have an event handler, for a quick test, you could put a 'Stop' in the event handler to stop like you are asking. If not you can put one in as already stated. Assertions are nice, I like them and use a lot of them - but here since you know the function and are working on this explicit problem a Stop should be good enough for your testing. Just don't save it for production code. Make a Test copy of the book.
Hope that helps.