diff --git a/_pytest/tmpdir.py b/_pytest/tmpdir.py
index a2a063200..b90e35948 100644
--- a/_pytest/tmpdir.py
+++ b/_pytest/tmpdir.py
@@ -52,10 +52,14 @@ class TempdirFactory:
                     basetemp.remove()
                 basetemp.mkdir()
             else:
-                # use a sub-directory in the temproot to speed-up
-                # make_numbered_dir() call
                 temproot = py.path.local.get_temproot()
-                rootdir = temproot.join('pytest-of-%s' % get_user())
+                user = get_user()
+                if user:
+                    # use a sub-directory in the temproot to speed-up
+                    # make_numbered_dir() call
+                    rootdir = temproot.join('pytest-of-%s' % user)
+                else:
+                    rootdir = temproot
                 rootdir.ensure(dir=1)
                 basetemp = py.path.local.make_numbered_dir(prefix='pytest-',
                                                            rootdir=rootdir)
@@ -68,15 +72,14 @@ class TempdirFactory:
 
 
 def get_user():
-    """Return the current user name, falling back to using "tox" as user name
-    because getpass relies on environment variables which might not be
-    available in a tox environment (see #1010).
+    """Return the current user name, or None if getuser() does not work
+    in the current environment (see #1010).
     """
     import getpass
     try:
         return getpass.getuser()
     except ImportError:
-        return 'tox'
+        return None
 
 # backward compatibility
 TempdirHandler = TempdirFactory
diff --git a/testing/test_tmpdir.py b/testing/test_tmpdir.py
index b04c73583..9bc0cf40a 100644
--- a/testing/test_tmpdir.py
+++ b/testing/test_tmpdir.py
@@ -145,4 +145,4 @@ def test_get_user(monkeypatch):
     from _pytest.tmpdir import get_user
     monkeypatch.delenv('USER', raising=False)
     monkeypatch.delenv('USERNAME', raising=False)
-    assert get_user() == 'tox'
+    assert get_user() is None