WiX Property: Directory Search: "Failed to access network location"

I am making an installer with a WiX 3.5 toolkit and I am facing a problem:

The installer should be able to determine if another program is present, and if so, add the DLL file to your directory. I use the following code to find out where the second program is installed:

<Property Id="FIND_INSTALLDIR" Value="[%ProgramFilesFolder]\PROGRAM">
    <RegistrySearch
        Id="INSTALLDIRSearch"
        Root="HKLM"
        Name="UninstallString"
        Type="file"
        Key="SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\PROGRAM"
    />
</Property>

The problem is that if the second program is not installed, the Windows installer gives me an error:

Could not access network location [%ProgramFilesFolder]\PROGRAM

I need to be able to handle this gracefully, though ... how can I recover from an error?