diff --git a/lib/iris/tests/unit/util/test__is_circular.py b/lib/iris/tests/unit/util/test__is_circular.py
new file mode 100644
index 0000000000..171368274b
--- /dev/null
+++ b/lib/iris/tests/unit/util/test__is_circular.py
@@ -0,0 +1,42 @@
+# (C) British Crown Copyright 2016, Met Office
+#
+# This file is part of Iris.
+#
+# Iris is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Lesser General Public License as published by the
+# Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Iris is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with Iris.  If not, see <http://www.gnu.org/licenses/>.
+"""Test function :func:`iris.util._is_circular`."""
+
+from __future__ import (absolute_import, division, print_function)
+from six.moves import (filter, input, map, range, zip)  # noqa
+
+# import iris tests first so that some things can be initialised before
+# importing anything else
+import iris.tests as tests
+
+import numpy as np
+
+from iris.util import _is_circular
+
+
+class Test(tests.IrisTest):
+    def test_simple(self):
+        data = np.arange(12) * 30
+        self.assertTrue(_is_circular(data, 360))
+
+    def test_negative_diff(self):
+        data = np.arange(12) * -30
+        self.assertTrue(_is_circular(data, 360))
+
+
+if __name__ == '__main__':
+    tests.main()
diff --git a/lib/iris/util.py b/lib/iris/util.py
index 7809dade10..3bd42424e0 100644
--- a/lib/iris/util.py
+++ b/lib/iris/util.py
@@ -1398,7 +1398,7 @@ def _is_circular(points, modulus, bounds=None):
         if len(points) > 1:
             diffs = list(set(np.diff(points)))
             diff = np.mean(diffs)
-            abs_tol = diff * 1.0e-4
+            abs_tol = np.abs(diff * 1.0e-4)
             diff_approx_equal = np.max(np.abs(diffs - diff)) < abs_tol
             if diff_approx_equal:
                 circular_value = (points[-1] + diff) % modulus