`help` leads to panic

[gu1p]

go1.13 linux/amd64 on Ubuntu 18.04.3

> help
panic: runtime error: index out of range [1] with length 0

goroutine 1 [running]:
github.com/takashabe/btcli/pkg/cmd/interactive.lazyDoHelp(0xb38000, 0xc0000cc010, 0xb3ab80, 0xc00031f050, 0xc0005dc410, 0x0, 0x0)
        /home/gustavoip/go/pkg/mod/github.com/takashabe/[email protected]/pkg/cmd/interactive/executor.go:66 +0x339
github.com/takashabe/btcli/pkg/cmd/interactive.doHelp(0xb38000, 0xc0000cc010, 0xb3ab80, 0xc00031f050, 0xc0005dc410, 0x0, 0x0)
        /home/gustavoip/go/pkg/mod/github.com/takashabe/[email protected]/pkg/cmd/interactive/executor.go:19 +0x6e
github.com/takashabe/btcli/pkg/cmd/interactive.(*Executor).Do(0xc0004e05c0, 0xc000324000, 0x4)
        /home/gustavoip/go/pkg/mod/github.com/takashabe/[email protected]/pkg/cmd/interactive/executor.go:49 +0x348
github.com/c-bata/go-prompt.(*Prompt).Run(0xc00013c280)
        /home/gustavoip/go/pkg/mod/github.com/c-bata/[email protected]/prompt.go:85 +0x80e
github.com/takashabe/btcli/pkg/cmd/interactive.(*CLI).Run(0xc0000acf00, 0xc0000d4000, 0x5, 0x5, 0x0)
        /home/gustavoip/go/pkg/mod/github.com/takashabe/[email protected]/pkg/cmd/interactive/interactive.go:65 +0x570
main.main()
        /home/gustavoip/go/pkg/mod/github.com/takashabe/[email protected]/cmd/btcli/btcli.go:22 +0xdf