I was getting this error seemingly out of the blue, and none of the usual search results made sense: I wasn't trying to make a list. Turns out, I'd improperly passed a doubly-nested child array to
React.createElement(). A la:
createElement( "div", divProps, aChild, moreChildren );
moreChildren was an array of nodes I'd made earlier. The fix was to remember to splat the new array:
createElement( "div", divProps, aChild, ...moreChildren );
Hooray for tired programming.