How can I add drivers for custom properties? Suppose that obj is a object. If I want to add a driver for its scale X, I can do

d = obj.driver_add("scale", 0).driver


but if obj has the custom property var, the following

d = obj.driver_add("var").driver


gives an error:

Traceback (most recent call last):
  File "&lt;blender_console&gt;", line 1, in &lt;module&gt;
TypeError: bpy_struct.driver_add(): property "var" not found


How do I add a driver in this case?
The datapath for a custom property is '["propname"]'

obj["prop"] = 34
obj.driver_add('["prop"]')