DS-009 The Pet Strengthening system isn't working.

To "Strengthening" a pet, you need to put the required item in the "Requirements" box. Example:

For the pet "Panda", you need the item "Black Iron Cutlass." I left-click on the item in my bag, the "Requirements" box turns green. Once the box is green, I try to drop the item with the right or left mouse button but it doesn't work. I tried with other pets and other items to make sure the problem wasn't just coming from the Panda.