One concern if there's a 2-diode full wave rectifier at the wall wart's input.
With AC, the input current will be split between both diodes. If you apply DC, one diode will see all of the input current, and it might not be able the handle all of the rated current itself.
If the diode burns...
Perhaps you are used to the negative terminal being grounded. With that arrangement, the voltage of the negative terminal is held at 0 Volts, and the positive terminal supplies positive voltage.
In your case, when the positive terminal is grounded, the positive terminal is held at 0 Volts...