Physics Work Problem?

A block of weight w sits on a frictionless inclined plane, which makes an angle θ with respect to the horizontal. A force of magnitude F, applied parallel to the incline, pulls the block up the plane at constant speed.

The block moves a distance L up the incline. The block does not stop after moving this distance but continues to move with constant speed. What is the total work Wtot done on the block by all forces? (Include only the work done after the block has started moving, not the work needed to start the block moving from rest.)

I said that it was the change in potential energy, mgh, which would be mgLsin@. But apparently the correct answer is 0. I can't see how this is the case. They justified it by saying there was no change in kinetic energy, which is true, but there is CLEARLY a change in potential energy. So how can no work have been done?