According to spec....
region-lines

The lines attribute, on getting, must return the WebVTT region lines of the WebVTT region that the VTTRegion object represents. On setting, if the new value is negative, then an IndexSizeError exception must be thrown. Otherwise, the WebVTT region lines must be set to the new value.

But according to webidl spec long type is ...
long-type

The long type is a signed integer type that has values in the range [−2147483648, 2147483647].

So if we are not accepting negative values for lines, then how about changing its type to "unsigned long"?