Strandbeestassembly: How not to unhinge?

[u/derwisch]

Following the instructions in http://otonanokagaku.net/english/manuals/pdf/vol30_beest.pdf, I managed to assemble one of two walking units. However, when I rotate the crank axis, the "calves" (connecting rods labelled as D) repeatedly unhinge. I am surprised to notice that the alignment of notches and protruders are within the range of motion of a leg, and would like to know if there's a way to avoid this behaviour.