panic on sync

[kapilt]

$ s3kor sync s3://bucket/prefix /local/dir

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x55d6e8]

goroutine 1 [running]:
main.NewSync(0x6ddd00, 0xffffe5b5c220, 0x5f, 0xffffe5b5c280, 0x31, 0x14, 0x0, 0x4000274000, 0x400005bf70, 0x0, ...)
        /home/ubuntu/s3kor/s3sync.go:314 +0x108
main.switchCommand(0x6ddd24, 0x4, 0x3, 0x3)
        /home/ubuntu/s3kor/s3kor.go:232 +0x278
main.main()
        /home/ubuntu/s3kor/s3kor.go:250 +0x1d0

arm64 fwiw, ubuntu 20.04 golang 1.13.8

[update] fwiw also tried 1.16