After just checking out jack1 from GitHub and trying to initialize the git submodules for headers, example-clients and tools I received the following error: $ git submodule update --init fatal: reference is not a tree: e74323c535876abca9a2293bd2ca426a3b91ff8b fatal: reference is not a tree: a2536c8b348bbfb21ec53db901e7992d7ffef474 fatal: reference is not a tree: 8e140b72de0231d129c6006db969f1dba4f1486b Unable to checkout 'e74323c535876abca9a2293bd2ca426a3b91ff8b' in submodule path 'example-clients' Unable to checkout 'a2536c8b348bbfb21ec53db901e7992d7ffef474' in submodule path 'jack' Unable to checkout '8e140b72de0231d129c6006db969f1dba4f1486b' in submodule path 'tools' After checking the referenced submodules, I found that the referenced commits _do not exist_ in the tree of these 3 repositories. The last change to the submodules was: Which introduced many changes. What happened here? Are there pending commits of these 3 repositorys that are not yet public or was there a rebase in these so that the history got rewritten? For the time being, I assume it is safe to assume that HEAD of master is a valid commit to check out? Best regards Markus