diff --git a/tests/utils.py b/tests/utils.py
index fc93c50..df76493 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -10,6 +10,7 @@ class UtilsTest(TestCase):
     def test_tau(self):
         self.assertEqual(tau([1, 0, 0, 0, 0, 0]), 1)
         self.assertEqual(tau([1, 1, 1, 1, 1, 1]), 0)
+        self.assertEqual(tau([0, 0, 0, 0, 0, 0]), None)
         self.assertEqual("%.2f" % tau([0, 8, 0, 0, 0, 2, 0, 2, 0, 0, 0, 0]), "0.95")  # example from Yanai et al. 2005
 
     def test_entropy(self):
diff --git a/utils/tau.py b/utils/tau.py
index 4722aaa..aa558c9 100644
--- a/utils/tau.py
+++ b/utils/tau.py
@@ -8,6 +8,9 @@ def tau(values):
     n = len(values)                   # number of values
     mxi = max(values)                 # max value
 
-    t = sum([1 - (x/mxi) for x in values])/(n - 1)
+    if mxi > 0:
+        t = sum([1 - (x/mxi) for x in values])/(n - 1)
 
-    return t
+        return t
+    else:
+        return None