diff --git a/elephant/statistics.py b/elephant/statistics.py
index 7b5fc2278..42c729b77 100644
--- a/elephant/statistics.py
+++ b/elephant/statistics.py
@@ -46,9 +46,11 @@ def isi(spiketrain, axis=-1):
     """
     if axis is None:
         axis = -1
-    intervals = np.diff(spiketrain, axis=axis)
-    if hasattr(spiketrain, 'waveforms'):
-        intervals = pq.Quantity(intervals.magnitude, units=spiketrain.units)
+    if isinstance(spiketrain, neo.SpikeTrain):
+        intervals = np.diff(
+            np.sort(spiketrain.times.view(pq.Quantity)), axis=axis)
+    else:
+        intervals = np.diff(np.sort(spiketrain), axis=axis)
     return intervals
 
 
diff --git a/elephant/test/test_statistics.py b/elephant/test/test_statistics.py
index 4b013fe79..f611c0b39 100644
--- a/elephant/test/test_statistics.py
+++ b/elephant/test/test_statistics.py
@@ -34,7 +34,8 @@ def setUp(self):
         self.targ_array_1d = self.targ_array_2d_1[0, :]
 
     def test_isi_with_spiketrain(self):
-        st = neo.SpikeTrain(self.test_array_1d, units='ms', t_stop=10.0)
+        st = neo.SpikeTrain(
+            self.test_array_1d, units='ms', t_stop=10.0, t_start=0.29)
         target = pq.Quantity(self.targ_array_1d, 'ms')
         res = es.isi(st)
         assert_array_almost_equal(res, target, decimal=9)