Comment 4 for bug 490787

This is not a bug in HPLIP.
This is happening, because gnome-scan is trying to set values for scan descriptor(s), which is null. This may be because of improper enumerating of device.
gnome-scan needs to enumerate the device to get exact values for each option descriptors Ex: 2=mode, 3=resolution, 5=contrast, 6=compression, etc.

These values for options may vary from device to device.

This is mainly happening, gnome-scan utility trying to set descriptor 10. But descriptor 10 is null. (not assigned to anything).
But it is supposed to assign scan source to descriptor 9.

You can cross verify the same with either "hp-scan" or be using "xsane" utility.
Both these utilities are working fine.

Thanks!
Naga Samrat Chowdary, Narla