I see where you're coming from. I would say it's the cause of the crash but not the root cause of the issue as a whole. Both obviously need to be fixed.
Fixing only the client side issue removes this crash entirely, but leaves one open for similar issues in the future.
Fixing the server side issue fixes the crash and related ones, but doesn't address the root cause of the faulty data being generated by the client.
As a backend dev, I'd point to this as the root cause, not the client sending invalid data.