runc exec error

# ./runc exec --help
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x4a561d]

goroutine 1 [running]:
github.com/codegangsta/cli.(*StringSlice).String(0x0, 0x0, 0x0)
	/root/src/github.com/opencontainers/runc2/Godeps/_workspace/src/github.com/codegangsta/cli/flag.go:110 +0x11d
flag.(*FlagSet).Var(0xc8200ba0c0, 0x7feb60aa6408, 0x0, 0x7cba98, 0x3, 0x821220, 0x19)
	/usr/local/go/src/flag/flag.go:766 +0x6e
github.com/codegangsta/cli.StringSliceFlag.Apply.func1(0x7cba98, 0x3)
	/root/src/github.com/opencontainers/runc2/Godeps/_workspace/src/github.com/codegangsta/cli/flag.go:147 +0x8b
github.com/codegangsta/cli.eachName(0x7cba98, 0x6, 0xc8200d1368)
	/root/src/github.com/opencontainers/runc2/Godeps/_workspace/src/github.com/codegangsta/cli/flag.go:54 +0x11f
github.com/codegangsta/cli.StringSliceFlag.Apply(0x7cba98, 0x6, 0x0, 0x821220, 0x19, 0x0, 0x0, 0xc8200ba0c0)
	/root/src/github.com/opencontainers/runc2/Godeps/_workspace/src/github.com/codegangsta/cli/flag.go:148 +0x415
github.com/codegangsta/cli.(*StringSliceFlag).Apply(0xc82009a3c0, 0xc8200ba0c0)
	<autogenerated>:23 +0xa0
github.com/codegangsta/cli.flagSet(0x7cbb38, 0x4, 0xc8200c8000, 0x6, 0xa, 0xc8200c8000)
	/root/src/github.com/opencontainers/runc2/Godeps/_workspace/src/github.com/codegangsta/cli/flag.go:45 +0x11a
github.com/codegangsta/cli.Command.Run(0x7cbb38, 0x4, 0x0, 0x0, 0x0, 0x0, 0x0, 0x836520, 0x28, 0x0, ...)
	/root/src/github.com/opencontainers/runc2/Godeps/_workspace/src/github.com/codegangsta/cli/command.go:60 +0x355
github.com/codegangsta/cli.(*App).Run(0xc8200b8000, 0xc82000a210, 0x3, 0x3, 0x0, 0x0)
	/root/src/github.com/opencontainers/runc2/Godeps/_workspace/src/github.com/codegangsta/cli/app.go:154 +0xb95
main.main()
	/root/src/github.com/opencontainers/runc2/main.go:103 +0x933

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
	/usr/local/go/src/runtime/asm_amd64.s:1721 +0x1

goroutine 18 [syscall]:
os/signal.loop()
	/usr/local/go/src/os/signal/signal_unix.go:22 +0x18
created by os/signal.init.1
	/usr/local/go/src/os/signal/signal_unix.go:28 +0x37