If you installed one of those Excel security patches issued by Microsoft earlier this week, you may want to double-check your math.

That's because a bug in the recent MS08-014 patch is causing Excel to return zeroes instead of the correct number when certain types of macros are run within the program.

The issue, which does not affect all Excel users, messes up calculations for Office Excel 2003 users who are grabbing data such as stock quotes or inventory levels from sources in real time and plugging them into an Excel array.

"If you have applications that leverage Real Time Data sources in Visual Basic for Applications functions, we recommend that you perform additional testing before initiating a wide deployment of the update," Microsoft said in an updated frequently asked-questions section of the bulletin on the MS08-014 update.

As a workaround, Microsoft recommends that users run affected Real Time Data functions on individual cells within the spreadsheet, rather than throughout the whole array.

Microsoft warned users of the problem late Thursday, and the company is working on a fix. The company's public relations agency could not immediately say Friday when this update might be released.

Ironically, Office 2003, Service Pack 3 users are not affected by the security flaw that is fixed with this patch, but Microsoft is still pushing this update out to these customers. Microsoft says that this is happening because the update files have higher version numbers than the existing files on Service Pack 3 systems.

Microsoft released four critical security patches on Tuesday, all related to Office. Of these, the MS08-014 update is one of the most important. It fixes a publicly disclosed flaw in Excel that hackers had been exploiting in online attacks since the middle of January.

The IDG News Service is a Network World affiliate.