@@ -14,6 +14,10 @@ class InvalidGitRepositoryError(Exception):
     """ Thrown if the given repository appears to have an invalid format.  """
 
 
+class WorkTreeRepositoryUnsupported(InvalidGitRepositoryError):
+    """ Thrown to indicate we can't handle work tree repositories """
+
+
 class NoSuchPathError(OSError):
     """ Thrown if a path could not be access by the system. """
 
 
@@ -4,7 +4,7 @@
 
 from gitdb.exc import (
     BadObject,
-    BadName
+    BadName,
 )
 from git.refs import SymbolicReference
 from git.objects import Object
@@ -16,6 +16,7 @@
     hex_to_bin,
     bin_to_hex
 )
+from git.exc import WorkTreeRepositoryUnsupported
 from git.compat import xrange
 
 
@@ -31,14 +32,20 @@ def touch(filename):
 
 def is_git_dir(d):
     """ This is taken from the git setup.c:is_git_directory
-    function."""
-    if isdir(d) and \
-            isdir(join(d, 'objects')) and \
-            isdir(join(d, 'refs')):
-        headref = join(d, 'HEAD')
-        return isfile(headref) or \
-            (os.path.islink(headref) and
-             os.readlink(headref).startswith('refs'))
+    function.
+
+    @throws WorkTreeRepositoryUnsupported if it sees a worktree directory. It's quite hacky to do that here,
+            but at least clearly indicates that we don't support it.
+            There is the unlikely danger to throw if we see directories which just look like a worktree dir,
+            but are none."""
+    if isdir(d):
+        if isdir(join(d, 'objects')) and isdir(join(d, 'refs')):
+            headref = join(d, 'HEAD')
+            return isfile(headref) or \
+                (os.path.islink(headref) and
+                 os.readlink(headref).startswith('refs'))
+        elif isfile(join(d, 'gitdir')) and isfile(join(d, 'commondir')) and isfile(join(d, 'gitfile')):
+            raise WorkTreeRepositoryUnsupported(d)
     return False
 
 
 
@@ -33,7 +33,10 @@
 )
 from git.repo.fun import touch
 from git.util import join_path_native
-from git.exc import BadObject
+from git.exc import (
+    BadObject,
+    WorkTreeRepositoryUnsupported
+)
 from gitdb.util import bin_to_hex
 from git.compat import string_types
 from gitdb.test.lib import with_rw_directory
@@ -45,6 +48,8 @@
 import itertools
 from io import BytesIO
 
+from nose import SkipTest
+
 
 class TestRepo(TestBase):
 
@@ -779,3 +784,16 @@ def test_is_ancestor(self):
         self.assertFalse(repo.is_ancestor("master", c1))
         for i, j in itertools.permutations([c1, 'ffffff', ''], r=2):
             self.assertRaises(GitCommandError, repo.is_ancestor, i, j)
+
+    @with_rw_directory
+    def test_work_tree_unsupported(self, rw_dir):
+        git = Git(rw_dir)
+        if git.version_info[:3] < (2, 5, 1):
+            raise SkipTest("worktree feature unsupported")
+
+        rw_master = self.rorepo.clone(join_path_native(rw_dir, 'master_repo'))
+        rw_master.git.checkout('HEAD~10')
+        worktree_path = join_path_native(rw_dir, 'worktree_repo')
+        rw_master.git.worktree('add', worktree_path, 'master')
+
+        self.failUnlessRaises(WorkTreeRepositoryUnsupported, Repo, worktree_path)