dsync fails with Thread Error

Attempting to `dsync` a directory from one S3 bucket to another results in the following errors:

```
$ s4cmd dsync --verbose s3://julialang/bin s3://julialang2/bin
[Exception] 'NoneType' object has no attribute '__getitem__'
s3://julialang/bin/checksums/julia-0.3.10.sha256 => s3://julialang2/bin/checksums/julia-0.3.10.sha256
s3://julialang/bin/checksums/julia-0.3.10.md5 => s3://julialang2/bin/checksums/julia-0.3.10.md5
s3://julialang/bin/checksums/julia-0.4.0-rc1.md5 => s3://julialang2/bin/checksums/julia-0.4.0-rc1.md5
s3://julialang/bin/checksums/julia-0.3.11.sha256 => s3://julialang2/bin/checksums/julia-0.3.11.sha256
s3://julialang/bin/checksums/julia-0.3.9.sha256 => s3://julialang2/bin/checksums/julia-0.3.9.sha256
s3://julialang/bin/checksums/julia-0.3.8.sha256 => s3://julialang2/bin/checksums/julia-0.3.8.sha256
s3://julialang/bin/checksums/julia-0.3.12.md5 => s3://julialang2/bin/checksums/julia-0.3.12.md5
s3://julialang/bin/checksums/julia-0.3.12.sha256 => s3://julialang2/bin/checksums/julia-0.3.12.sha256
s3://julialang/bin/checksums/julia-0.3.9.md5 => s3://julialang2/bin/checksums/julia-0.3.9.md5
s3://julialang/bin/checksums/julia-0.4.0-rc2.md5 => s3://julialang2/bin/checksums/julia-0.4.0-rc2.md5
s3://julialang/bin/checksums/julia-0.3.11.md5 => s3://julialang2/bin/checksums/julia-0.3.11.md5
s3://julialang/bin/checksums/julia-0.4.0-rc3.md5 => s3://julialang2/bin/checksums/julia-0.4.0-rc3.md5
s3://julialang/bin/checksums/julia-0.4.0-rc1.sha256 => s3://julialang2/bin/checksums/julia-0.4.0-rc1.sha256
s3://julialang/bin/checksums/julia-0.3.8.md5 => s3://julialang2/bin/checksums/julia-0.3.8.md5
s3://julialang/bin/checksums/julia-0.4.0-rc2.sha256 => s3://julialang2/bin/checksums/julia-0.4.0-rc2.sha256
[Thread Failure] 'NoneType' object has no attribute '__getitem__'
```

Reproduce the reported `s4cmd dsync --verbose` command between the two S3 buckets and capture where the `Thread Failure` occurs. Trace the dsync and thread-error path from that command; done means the directory copy completes without the reported NoneType exception.

Written by the indexing model from the issue text.