Setting the INSTALLDIR programatically in InstallShield

Question

I am trying to set the change the INSTALLDIR programatically in InstallShield v8.02 (cannot do anything about the version).

My code crashes with an exception "Unknown error" on the third line of this code (C#)

String originalInstallDir = testProject.get_INSTALLDIR();
String installDir = originalInstallDir.Replace("Company", "Company Test");
testProject.set_INSTALLDIR(ref(installDir));

Any suggestions?