Go into the network of your background object,
disconnect the image map shader from the background's surface shader,
create a a default shader,
connect the image map shader with the colour function input and the luminosity input of the new default shader,
set the value for diffuse color and luminosity to 1,
IMPORTANT: assign a camera to the image map shader, when it's set to "through camera"! Otherwise TG doesn't know where to project through.
And voilá, there's your background