pcottle/learnGitBranching · error · GitError

fatal: not a branch: ' + oldName

Error message

fatal: not a branch: ' + oldName

What it means

Duplicate of error 65: the 'fatal: not a branch: <name>' GitError thrown by renameBranch when the old name does not resolve to a branch. The listed SOURCE message string is just the expression used to build the message; the condition is !target || target.get('type') !== 'branch'.

Source

Thrown at src/js/git/index.js:2821

  }

  // then join
  throw new CommandResult({
    msg: foundTag + '-' + numAway.length + '-g' + startCommit.get('id')
  });
};

GitEngine.prototype.renameBranch = function(oldName, newName, force) {
  var target = this.resolveID(oldName);

  if (!target || target.get('type') !== 'branch') {
    throw new GitError({
      msg: intl.todo('fatal: not a branch: ' + oldName)
    });
  }

  if (target.getIsRemote()) {
    throw new GitError({
      msg: intl.str('git-error-remote-branch')
    });
  }

  newName = this.validateBranchName(newName);

  if (this.doesRefExist(newName)) {
    if (!force) {
      throw new GitError({
        msg: intl.todo("fatal: A branch named '" + newName + "' already exists.")
      });
    }
    var existing = this.resolveID(newName);
    if (existing.get('type') !== 'branch') {
      throw new GitError({
        msg: intl.str('git-error-options')
      });
    }

View on GitHub (pinned to 5b09d0ff96)

Solutions

  1. Validate the ref type before calling renameBranch
  2. Correct the branch name
  3. Delete-then-create the branch if the intent was to move a non-branch ref
Defensive patterns

Strategy: validation

Validate before calling

var t = engine.resolveID(oldName); if (!t || t.get('type') !== 'branch') return;

Type guard

function isBranch(engine, name) { var r = engine.resolveID(name); return !!r && r.get('type') === 'branch'; }

Prevention

When it happens

Trigger: Same as 65: renameBranch with an oldName that is null, a tag, a commit, or otherwise not a branch ref.

Common situations: Misspelled branch names, renaming tags, passing HEAD.

Related errors

AI-assisted analysis of pcottle/learnGitBranching@5b09d0ff96 (2026-08-27). Data as JSON: /api/errors/af0422c4d22f6627. Report an issue: GitHub.