viFindRsrc using VI_ATTR_USB_SERIAL_NUM

I'm trying to get the VISA resource string for a USB/RAW device based on the serial number.  If I do

viFindRsrc(hRM, "/USB?*RAW{VI_ATTR_USB_SERIAL_NUM == 123456/789}", VI_NULL, &lCount, szBuffer);

I get VI_ERROR_RSRC_NFOUND.  However, if I just use "/USB?*RAW" as the resource string, then go through the list, open a session to each device, and use

viGetAttribute(hSession, VI_ATTR_USB_SERIAL_NUM, szBuffer);

I can find the correct device by looking at szBuffer.  Am I doing something wrong in the viFindRsrc call?  Do I need to escape the / in the serial number, for instance?  Or is going through the full list the only way to find the device?

Thanks in advance for your help.