This validation does not work if the userId is null:
https://github.com/segmentio/analytics-ios/blob/master/Segment/Classes/SEGAnalytics.m#L272

But here it is checked: https://github.com/segmentio/analytics-ios/blob/master/Segment/Classes/SEGAnalytics.m#L293

It is defined as optional on https://segment.com/docs/connections/sources/catalog/libraries/mobile/ios/#identify

I found this because the flutter implementation of segment (claimsforce-gmbh/flutter-segment#59) allowed userId to be null, but it throws this error:

-[NSNull length]: unrecognized selector sent to instance 0x1db450558