I bought Leviton’s “black and white professional” 20 amp 125V nylon connector(#5369 NEMA 5-20R) and plug(#5366 NEMA 5-20P)for my dishwasher. Both the dishwasher powerline from the panel box and the dishwasher line have solid 12 AWG wire (20 amp). But the instructions to both the connector and the plug both emphatically say “DO NOT use with solid wire”. Why is this? Is it dangerous?