Normally there is a metal slug with a spring behind it that fits in the hole in the coupler. I am guessing that has now fallen out. You have to locate that first. You may or may not have remove the hinge rivet to complete the repair.
The parts will look similar to these.
https://www.ebay.com/itm/Lione...8:g:cCgAAOSws6tenHBV
Pete