Commit indexer is not working for repository root

I am creating Virtual File system for my application. As input I am getting file paths or directory paths. I found that app works perfectly for any paths except for root.

Technically speaking I cannot get TreeEntry for repo root directory.

            Commit commit = GetCommit(commitHash);

            // this works
            var file = commit["foo.txt"];
            var dir1 = commit["var/logs"];
            var dir2 = commit["var"];

            // this doesn't work
            ver rootDir = commit["/"]; // returns null

Reproduce the difference between commit["foo.txt"], commit["var"], and commit["/"] using the commit indexer and TreeEntry behavior described here. Trace how the indexer resolves the repository root, then verify that a root lookup returns the expected TreeEntry without breaking file or nested-directory lookups.