property undefined identifier

[gopenguins]

I have the following script in my Basic MSI project that is called by a custom action:

#include "ifx.h"
export prototype SetFileExistsProperties(HWND);

function SetFileExistsProperties(hMSI)
    NUMBER nResult;
    STRING sApp;
begin   
        sApp="C:\\file.tmp";
	nResult = Is(FILE_EXISTS, sApp);
	if (nResult = 1) then
		MsiSetProperty(ISMSI_HANDLE,APPEXISTS,"Yes")
        endif;
end;

Where APPEXISTS is defined in the property table with an initial value set to "No"

When I compile I get the following error:
APPEXISTS: undefiend identifier
APPEXISTS: string value required

Please help! What am I missing?

[RobertDickau]

The property name in MsiSetProperty (and other MSI API functions) should be in quotation marks:

MsiSetProperty(hInstall, "PropertyName", "PropertyValue");