diff --git a/package/MDAnalysis/auxiliary/XVG.py b/package/MDAnalysis/auxiliary/XVG.py
index befe94c7ee7..4bf2c083c21 100644
--- a/package/MDAnalysis/auxiliary/XVG.py
+++ b/package/MDAnalysis/auxiliary/XVG.py
@@ -68,7 +68,6 @@
 """
 from __future__ import absolute_import
 
-from six.moves import range
 from six import raise_from
 
 import numbers
@@ -77,6 +76,7 @@
 from . import base
 from ..lib.util import anyopen
 
+
 def uncomment(lines):
     """ Remove comments from lines in an .xvg file
 
diff --git a/testsuite/MDAnalysisTests/auxiliary/base.py b/testsuite/MDAnalysisTests/auxiliary/base.py
index 61395a98f58..93445f7f105 100644
--- a/testsuite/MDAnalysisTests/auxiliary/base.py
+++ b/testsuite/MDAnalysisTests/auxiliary/base.py
@@ -227,6 +227,11 @@ def test_time_selector(self, ref):
         for i, val in enumerate(reader):
             assert val.time == ref.select_time_ref[i], "time for step {} does not match".format(i)
 
+    def test_time_non_constant_dt(self, reader):
+        reader.constant_dt = False
+        with pytest.raises(ValueError, match="If dt is not constant, must have a valid time selector"):
+            reader.time
+
     def test_time_selector_manual(self, ref):
         reader = ref.reader(ref.testdata,
                             time_selector = ref.time_selector)
@@ -367,7 +372,7 @@ def test_step_to_frame_out_of_bounds(self, reader, ref):
     def test_step_to_frame_no_time_diff(self, reader, ref):
 
         ts = mda.coordinates.base.Timestep(0, dt=ref.dt)
-        
+
         for idx in range(reader.n_steps):
 
             assert reader.step_to_frame(idx, ts) == idx
@@ -385,6 +390,19 @@ def test_step_to_frame_time_diff(self, reader, ref):
             assert frame == idx
             np.testing.assert_almost_equal(time_diff, idx * 0.1)
 
+    def test_go_to_step_fail(self, reader):
+
+        with pytest.raises(ValueError, match="Step index [0-9]* is not valid for auxiliary"):
+            reader._go_to_step(reader.n_steps)
+
+    @pytest.mark.parametrize("constant", [True, False])
+    def test_set_constant_dt(self, reader, constant):
+
+        reader.constant_dt = constant
+
+        assert reader.constant_dt == constant
+
+
 def assert_auxstep_equal(A, B):
     if not isinstance(A, mda.auxiliary.base.AuxStep):
         raise AssertionError('A is not of type AuxStep')
diff --git a/testsuite/MDAnalysisTests/auxiliary/test_core.py b/testsuite/MDAnalysisTests/auxiliary/test_core.py
new file mode 100644
index 00000000000..9a624be1c6f
--- /dev/null
+++ b/testsuite/MDAnalysisTests/auxiliary/test_core.py
@@ -0,0 +1,43 @@
+# -*- Mode: python; tab-width: 4; indent-tabs-mode:nil; coding:utf-8 -*-
+# vim: tabstop=4 expandtab shiftwidth=4 softtabstop=4 fileencoding=utf-8
+#
+# MDAnalysis --- https://www.mdanalysis.org
+# Copyright (c) 2006-2017 The MDAnalysis Development Team and contributors
+# (see the file AUTHORS for the full list of names)
+#
+# Released under the GNU Public Licence, v2 or any higher version
+#
+# Please cite your use of MDAnalysis in published work:
+#
+# R. J. Gowers, M. Linke, J. Barnoud, T. J. E. Reddy, M. N. Melo, S. L. Seyler,
+# D. L. Dotson, J. Domanski, S. Buchoux, I. M. Kenney, and O. Beckstein.
+# MDAnalysis: A Python package for the rapid analysis of molecular dynamics
+# simulations. In S. Benthall and S. Rostrup editors, Proceedings of the 15th
+# Python in Science Conference, pages 102-109, Austin, TX, 2016. SciPy.
+# doi: 10.25080/majora-629e541a-00e
+#
+# N. Michaud-Agrawal, E. J. Denning, T. B. Woolf, and O. Beckstein.
+# MDAnalysis: A Toolkit for the Analysis of Molecular Dynamics Simulations.
+# J. Comput. Chem. 32 (2011), 2319--2327, doi:10.1002/jcc.21787
+#
+
+from __future__ import absolute_import
+
+import MDAnalysis as mda
+
+import pytest
+
+
+def test_get_auxreader_for_none():
+    with pytest.raises(ValueError, match="Must provide either auxdata or format"):
+        mda.auxiliary.core.get_auxreader_for()
+
+
+def test_get_auxreader_for_wrong_auxdata():
+    with pytest.raises(ValueError, match="Unknown auxiliary data format for auxdata:"):
+        mda.auxiliary.core.get_auxreader_for(auxdata="test.none")
+
+
+def test_get_auxreader_for_wrong_format():
+    with pytest.raises(ValueError, match="Unknown auxiliary data format"):
+        mda.auxiliary.core.get_auxreader_for(format="none")
diff --git a/testsuite/MDAnalysisTests/auxiliary/test_xvg.py b/testsuite/MDAnalysisTests/auxiliary/test_xvg.py
index 2b4791c0744..a7ecf3a7531 100644
--- a/testsuite/MDAnalysisTests/auxiliary/test_xvg.py
+++ b/testsuite/MDAnalysisTests/auxiliary/test_xvg.py
@@ -33,6 +33,7 @@
 
 from MDAnalysisTests.datafiles import AUX_XVG, XVG_BAD_NCOL, XVG_BZ2
 from MDAnalysisTests.auxiliary.base import (BaseAuxReaderTest, BaseAuxReference)
+from MDAnalysis.auxiliary.XVG import XVGStep
 
 
 class XVGReference(BaseAuxReference):
@@ -52,6 +53,29 @@ def __init__(self):
         self.select_data_ref = [self.format_data([2*i, 2**i]) for i in range(self.n_steps)]
 
 
+class TestXVGStep():
+
+    @staticmethod
+    @pytest.fixture()
+    def step():
+        return XVGStep()
+
+    def test_select_time_none(self, step):
+
+        st = step._select_time(None)
+
+        assert st is None
+
+    def test_select_time_invalid_index(self, step):
+        with pytest.raises(ValueError, match="Time selector must be single index"):
+            step._select_time([0])
+
+    def test_select_data_none(self, step):
+
+        st = step._select_data(None)
+
+        assert st is None
+
 class TestXVGReader(BaseAuxReaderTest):
     @staticmethod
     @pytest.fixture()