"error: failed to read cfg item 3704: Other error" with empty serial number

[emaste]

I have a cp2102 USB-serial dongle with no serial number set, and cp210x-cfg reports an error:

# ./cp210x-cfg
ID 10c4:ea60 @ bus 000, dev 003: CP2102 USB to UART Bridge Controller
Model: CP2102
Vendor ID: 10c4
Product ID: ea60
Name: CP2102 USB to UART Bridge Controller
error: failed to read cfg item 3704: Other error

I can set a serial number:

# ./cp210x-cfg -S $(pwgen -s 10)
ID 10c4:ea60 @ bus 000, dev 003: CP2102 USB to UART Bridge Controller
Model: CP2102
Vendor ID: 10c4
Product ID: ea60
Name: CP2102 USB to UART Bridge Controller
Serial: 8OoOE8NmD5

And I can tell cp210x-cfg to set the serial number back to an empty string, which returns the error again:

# ./cp210x-cfg -S ""
ID 10c4:ea60 @ bus 000, dev 003: CP2102 USB to UART Bridge Controller
Model: CP2102
Vendor ID: 10c4
Product ID: ea60
Name: CP2102 USB to UART Bridge Controller
error: failed to read cfg item 3704: Other error

But the Python cp210x-program does report the empty serial number, without error:

# python2 ./cp210x-program
[usb device]
product_string = CP2102 USB to UART Bridge Controller
serial_number = 
...

So it looks like everything functions correctly, except for emitting the error message.