What steps will reproduce the problem?
1. Run

package main

import (
      "fmt"
      "net"
)

func main() {
      _, err := net.Listen("tcp", "")
      fmt.Println(err)
}

What is the expected output? What do you see instead?

Expected:
missing port in address

Got:
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x38 pc=0x43750e]

goroutine 1 [running]:
net.favoriteAddrFamily(0x4d2624, 0x7f1c00000003, 0x0, 0x0, 0x0, ...)
    /build/src/release-build/src/pkg/net/ipsock_posix.go:100 +0xc3
net.internetSocket(0x4d2624, 0x7f1c00000003, 0x0, 0x0, 0x0, ...)
    /build/src/release-build/src/pkg/net/ipsock_posix.go:127 +0xd9
net.ListenTCP(0x4d2624, 0x7473696c00000003, 0x0, 0x70637400000003, 0x0, ...)
    /build/src/release-build/src/pkg/net/tcpsock_posix.go:302 +0xc2
net.Listen(0x4d2624, 0x70637400000003, 0x4d17e4, 0x0, 0x47fdf8, ...)
    /build/src/release-build/src/pkg/net/dial.go:188 +0x1e3
main.main()
    /tmp/test.go:9 +0x3d

goroutine 2 [syscall]:
created by runtime.main
    /build/src/release-build/src/pkg/runtime/proc.c:221
exit status 2