I found that the error only occurs in Internet Explorer 11.
In Chrome it works well.
That... actually helps. I re-created the error, and now see it in English.
Object doesn't support property or method 'sign'
Apparently Internet Explorer doesn't support Math.sign(). At all. This is the kind of thing that makes web developers hate IE.
This is from MSDN:
Supported in Microsoft Edge (Edge browser). Also supported in Store apps (Microsoft Edge on Windows 10). See Version Information.
Not supported in the following document modes: Quirks, Internet Explorer 6 standards, Internet Explorer 7 standards, Internet Explorer 8 standards, Internet Explorer 9 standards, Internet Explorer 10 standards, Internet Explorer 11 standards. Not supported in Windows 8.1.
I'm not really sure what I'm gonna do about that. I'm leaning heavily towards "screw Microsoft and their non-standards-compliant browser".