PaulS:
Do compound operators not work with String objects?
They do. The String class is a resource hog, though. I'd guess that you are right at the limit of not having enough memory.
Thanks for replying. Well, I realize the String class is a resource hog, but I'm curious why it doesn't work in this case. Surely the Mega has enough memory for a simple string in an example sketch? Besides, changing the compound operator to the regular concat operator works.