When would it be nil?

It's an internal function. What is calling it with a nil value?

Please provide a way to reproduce the problem.

Thanks for your quick reply, I encountered this issue when testing docker on a device with multiple interfaces. Some of them where up but with no ip adresses. The call to networkNumberAndMask was made by the String function of ip.go. However, as this internal function is also used by Contains in ip.go, I think it should be fixed inside networkNumberAndMask. I have not trace where docker made the call to String as I thought that the problem was inside Go. If needed, I can send more traces.

Sounds like some code forgot an error check.

It's not valid to call String or Contains on a nil *IPNet.