Why doesn't the _issi.cfg file work in Inno Setup?

I wrote this code to add the splash screen before my installer starts:

[ISSI]

#define ISSI_Splash "C:\ABoffice\Install\InnoSetupProject\Images\client.bmp"
#define ISSI_Splash_T 5
#define ISSI_Splash_X 500
#define ISSI_Splash_Y 220
//#define ISSI_IncludePath "C:\ISSI"
#include ISSI_IncludePath+"\_issi.isi"

[Setup]
AppName=Client AB OFFICE 
AppVersion=5.0
DefaultDirName={sd}\AB_Office\Client

The instruction manual says that I can configure my ISSI installation by setting some presets in the "_issi.cfg" file located in the ISSI folder. Here's how my "_issi.cfg" file looks like:

[ISSI Configuration]
#define ISSI_IncludePath "C:\ISSI"
;#define ISSI_Constants "YYMDHMS"
;#define ISSI_ConstantsSeperator "."
;#define ISSI_Compression
;#define ISSI_BeveledLabel

[Setup]
OutputDir=C:\Inno Setup Output

But when I compile my script I get this error:

[ISPP] Undeclared identifier: "ISSI_IncludePath".

What do I need to add to my code to be able to read from the _issi.cfg?

You are setting ISSI_IncludePath in the _issi.cfg.

And you then try to include the _issi.cfg into your .iss script by referring to it using the ISSI_IncludePath.

That's a circular dependency.

Maybe you wanted to set the ISSI_IncludePath in your iss file:

#define ISSI_IncludePath "C:\ISSI"

#include ISSI_IncludePath+"\_issi.isi"

Then you will probably have you remove the #define ISSI_IncludePath from the _issi.cfg.