Comment 4 for bug 784209

as discussed on irc, it's because of the default background value:

$ gsettings get org.gnome.desktop.background picture-uri
'file:///usr/share/themes/Adwaita/backgrounds/stripes.jpg'

it doesn't exist unless you have gnome-themes-standard installed.

The 2 workarounds are:

1/ change that key to a valid URI (still starting with file://)

or

2/ install gnome-themes-standard

The real fix is to change the default value to match an existing Ubuntu theme.