My steamvac f5870-900 trigger mechanism isn't...

Question

I had a rattle in the handle portion of the steam vac so I took the handle apart (by reversing the assembly guide) and a small plastic piece fell out. I can not tell where it should go - there doesn't seem to be a place where it broke as the piece is intact. It has to be important 'cuz if you put the water tank on you get a flood!

Your assistance is appreciated