Intermittent MergeFetchHeadNotFoundException

[miloush]

I am getting intermittent

LibGit2Sharp.MergeFetchHeadNotFoundException: The current branch is configured to merge with the reference 'refs/heads/main' from the remote, but this reference was not fetched.
   at LibGit2Sharp.Repository.MergeFetchedRefs(Signature merger, MergeOptions options) in /_/LibGit2Sharp/Repository.cs:line 1268
   at LibGit2Sharp.Commands.Pull(Repository repository, Signature merger, PullOptions options) in /_/LibGit2Sharp/Commands/Pull.cs:line 37

when calling Commands.Pull on https://github.com/unicode-org/unicodetools. Clearly the remote ref (main branch) has been there for a while and is not going anywhere. Trying pull again usually succeeds without an exception.

Any other reason why this error would happen, or is there some logging I could turn on to figure out what's going on? For example, what refs does it see (my guess would be none). I don't have a reliable repro, never happened on my local machine, happens uncomfortably often on a server machine.

The clone is for reading only, the point is to just keep it up to date, i.e. there is no local changes to merge ever, so I use these options:

pullOptions.MergeOptions = new MergeOptions();
pullOptions.MergeOptions.SkipReuc = true;
pullOptions.MergeOptions.FastForwardStrategy = FastForwardStrategy.FastForwardOnly;