We’re getting some other reports that this may be version dependent and introduced in 1.13.x. We are so far unable to reproduce on our end.
@jie - Could you try downgrading to 1.12.0 or 1.11.0 and see if the error remains? If so, then this a bug with something we did recently rather than the dash.py-style of issue that we usually see.
Thanks to everyone who is helping out diagnose this issue!