starting EXE through MSI and error when running MSI

[Tornado]

Hi,
I made a MSI-package of a VB-project with Windows Installer.
After completing of the installation I need to run an EXE to which
installs additional components.
Currently I'm trying to store them in same folder as the MSI-file and
call it with a CustomAction.
The CustomAction looks like this:
Action: RunEXE
Type: 2
Source:
Target: [SourceDir]Modules.exe

I don't know if this is the correct or ideal way to do this, but after
adding this row and running the MSI-file I get the following error:
"This installation package could not be opened. Verify that the
package exists and that you can access it, or contact the application
vendor to verify that this is a valid Windows Installer package."
In the help from the Windows Installer this is known as
ERROR_INSTALL_PACKAGE_OPEN_FAILED 1619 but I don't see a way to solve
this.

Does someone know how I can solve this error and if I'm calling my EXE
the right way?

Thanks,
Johan