Error 1720 in custom action

I'm trying to use a VBScript custom action to alter the [INSTALLDIR] property.
I've got the script set to run 'After Initialization (before first dialog)'.
Even when I have the script with just 'MsgBox "Script is running"', I get the following error:

Error 1720. There is a problem with this Windows
Installer package. A script required for this install to
complete count not be run. Contact your support
personnel or package vendor. Custom action
NewCustomAction1 script error -2146827256, Micros...

Help!