An instruction that used to work now crashes my app!!

A line of vba code in an Excel application I'm developing - that used to run and execute perfectly - now causes a Fatal Error and shuts down my application when the line executes. This happens whether I execute the code in real time or whether I step through the code using the F8 key.

I'm assuming the line of code that is crashing my app is (I hope I have used the code embed feature correctly)

ActiveWorkbook.Close True

because when I step through the code (F8) everthing runs fine until I execute this line. Anyone know why this sort of thing happens?

The ActiveWorkbook reference is a workbook that I opened using my vba code.

I'm using Excel 97 (XP, SP2) and Excel 2007 (Vista) and my app will crash on either.

Thank you,

Stu