I'm not sure if this is correct behavior or a bug, but if I append an empty string it causes an error. So $('#someId').append('') causes an error. In my case, I'm actually supplying a variable to append() but this variable could be an empty string. I don't see why appending nothing would cause an error, which is why I think it's a bug.
Anyway, for now, I'm just going to check if the variable is empty before calling append().