diff --git a/IPython/html/services/notebooks/filenbmanager.py b/IPython/html/services/notebooks/filenbmanager.py
index d313d63..c905ae6 100644
--- a/IPython/html/services/notebooks/filenbmanager.py
+++ b/IPython/html/services/notebooks/filenbmanager.py
@@ -131,28 +131,29 @@ class FileNotebookManager(NotebookManager):
         path = self.get_os_path(name, path)
         return os.path.isfile(path)
 
-    def get_os_path(self, fname, path=None):
-        """Return a full path to a notebook with the os.sep as the separator.
+    def get_os_path(self, fname, path='/'):
+        """Given a notebook name and a server URL path, return its file system
+        path.
         
         Parameters
         ----------
         fname : string
             The name of a notebook file with the .ipynb extension
         path : string
-            The relative path (with '/' as separator) to the named notebook.
+            The relative URL path (with '/' as separator) to the named
+            notebook.
             
         Returns
         -------
-        path : 
-            A path that combines notebook_dir (location where server started),
-            the relative path, and the filename with the current operating
-            system's url.
+        path : string
+            A file system path that combines notebook_dir (location where
+            server started), the relative path, and the filename with the
+            current operating system's url.
         """
-        if path is None:
-            path = '/'
         parts = path.split('/')
         parts = [p for p in parts if p != ''] # remove duplicate splits
-        path = os.sep.join([self.notebook_dir] + parts + [fname])
+        parts += [fname]
+        path = os.path.join(self.notebook_dir, *parts)
         return path
 
     def read_notebook_object_from_path(self, path):
diff --git a/IPython/html/services/notebooks/tests/test_nbmanager.py b/IPython/html/services/notebooks/tests/test_nbmanager.py
index 91294bb..ec3acb2 100644
--- a/IPython/html/services/notebooks/tests/test_nbmanager.py
+++ b/IPython/html/services/notebooks/tests/test_nbmanager.py
@@ -14,14 +14,14 @@ class TestFileNotebookManager(TestCase):
 
     def test_nb_dir(self):
         with TemporaryDirectory() as td:
-            km = FileNotebookManager(notebook_dir=td)
-            self.assertEqual(km.notebook_dir, td)
+            fm = FileNotebookManager(notebook_dir=td)
+            self.assertEqual(fm.notebook_dir, td)
 
     def test_create_nb_dir(self):
         with TemporaryDirectory() as td:
             nbdir = os.path.join(td, 'notebooks')
-            km = FileNotebookManager(notebook_dir=nbdir)
-            self.assertEqual(km.notebook_dir, nbdir)
+            fm = FileNotebookManager(notebook_dir=nbdir)
+            self.assertEqual(fm.notebook_dir, nbdir)
 
     def test_missing_nb_dir(self):
         with TemporaryDirectory() as td:
@@ -37,22 +37,21 @@ class TestFileNotebookManager(TestCase):
         # separators.
         with TemporaryDirectory() as td:
             nbdir = os.path.join(td, 'notebooks')
-            km = FileNotebookManager(notebook_dir=nbdir)
-            path = km.get_os_path('test.ipynb', '/path/to/notebook/')
-            self.assertEqual(path, km.notebook_dir+os.sep+'path'+os.sep+'to'+os.sep+
-                'notebook'+os.sep+'test.ipynb')
-
-        with TemporaryDirectory() as td:
-            nbdir = os.path.join(td, 'notebooks')
-            km = FileNotebookManager(notebook_dir=nbdir)
-            path = km.get_os_path('test.ipynb', None)
-            self.assertEqual(path, km.notebook_dir+os.sep+'test.ipynb')
-
-        with TemporaryDirectory() as td:
-            nbdir = os.path.join(td, 'notebooks')
-            km = FileNotebookManager(notebook_dir=nbdir)
-            path = km.get_os_path('test.ipynb', '////')
-            self.assertEqual(path, km.notebook_dir+os.sep+'test.ipynb')
+            fm = FileNotebookManager(notebook_dir=nbdir)
+            path = fm.get_os_path('test.ipynb', '/path/to/notebook/')
+            rel_path_list =  '/path/to/notebook/test.ipynb'.split('/')
+            fs_path = os.path.join(fm.notebook_dir, *rel_path_list)
+            self.assertEqual(path, fs_path)
+
+            fm = FileNotebookManager(notebook_dir=nbdir)
+            path = fm.get_os_path('test.ipynb')
+            fs_path = os.path.join(fm.notebook_dir, 'test.ipynb')
+            self.assertEqual(path, fs_path)
+
+            fm = FileNotebookManager(notebook_dir=nbdir)
+            path = fm.get_os_path('test.ipynb', '////')
+            fs_path = os.path.join(fm.notebook_dir, 'test.ipynb')
+            self.assertEqual(path, fs_path)
 
 class TestNotebookManager(TestCase):
     def test_named_notebook_path(self):