diff --git a/common/lib/xmodule/xmodule/fields.py b/common/lib/xmodule/xmodule/fields.py
index e33bc02b0f..878505722f 100644
--- a/common/lib/xmodule/xmodule/fields.py
+++ b/common/lib/xmodule/xmodule/fields.py
@@ -150,7 +150,7 @@ class IsoTime(Field):
         try:
             obj_time = time.strptime(value, '%H:%M:%S')
         except ValueError as e:
-            raise e(
+            raise ValueError(
                 "Incorrect IsoTime value {} was set in XML or serialized."
                 "Original parse message is {}".format(value, e.message)
             )
@@ -174,7 +174,11 @@ class IsoTime(Field):
         if isinstance(value, float):
             return datetime.timedelta(seconds=value)
 
-        return self._isotime_to_timedelta(value)
+        if isinstance(value, basestring):
+            return self._isotime_to_timedelta(value)
+
+        msg = "IsoTime Field {0} has bad value '{1}'".format(self._name, value)
+        raise TypeError(msg)
 
     def to_json(self, value):
         """
@@ -194,9 +198,12 @@ class IsoTime(Field):
                 value = 86400
             return str(datetime.timedelta(seconds=value))
 
-        if value.total_seconds() > 86400:  # sanity check
-            raise ValueError(
-                "IsoTime max value is 23:59:59=86400 seconds"
-                "but {} seconds is passed".format(value.total_seconds())
-            )
-        return str(value)
+        if isinstance(value, datetime.timedelta):
+            if value.total_seconds() > 86400:  # sanity check
+                raise ValueError(
+                    "IsoTime max value is 23:59:59=86400.0 seconds, "
+                    "but {} seconds is passed".format(value.total_seconds())
+                )
+            return str(value)
+
+        raise TypeError("IsoTime: cannot convert {!r} to json".format(value))
diff --git a/common/lib/xmodule/xmodule/tests/test_fields.py b/common/lib/xmodule/xmodule/tests/test_fields.py
index 8453adaa20..99fc1b9626 100644
--- a/common/lib/xmodule/xmodule/tests/test_fields.py
+++ b/common/lib/xmodule/xmodule/tests/test_fields.py
@@ -2,7 +2,7 @@
 import datetime
 import unittest
 from django.utils.timezone import UTC
-from xmodule.fields import Date, Timedelta
+from xmodule.fields import Date, Timedelta, IsoTime
 from xmodule.timeinfo import TimeInfo
 import time
 
@@ -116,3 +116,54 @@ class TimeInfoTest(unittest.TestCase):
         timeinfo = TimeInfo(due_date, grace_pd_string)
         self.assertEqual(timeinfo.close_date,
             due_date + Timedelta().from_json(grace_pd_string))
+
+
+class IsoTimeTest(unittest.TestCase):
+
+    delta = IsoTime()
+
+    def test_from_json(self):
+        self.assertEqual(
+            IsoTimeTest.delta.from_json('0:05:07'),
+            datetime.timedelta(seconds=307)
+        )
+
+        self.assertEqual(
+            IsoTimeTest.delta.from_json(100.0),
+            datetime.timedelta(seconds=100)
+        )
+        self.assertEqual(
+            IsoTimeTest.delta.from_json(None),
+            datetime.timedelta(seconds=0)
+        )
+
+        with self.assertRaises(TypeError):
+            IsoTimeTest.delta.from_json(1234)  # int
+
+        with self.assertRaises(ValueError):
+            IsoTimeTest.delta.from_json("77:77:77")
+
+    def test_to_json(self):
+        self.assertEqual(
+            "1:02:03",
+            IsoTimeTest.delta.to_json(datetime.timedelta(seconds=3723))
+        )
+        self.assertEqual(
+            "00:00:00",
+            IsoTimeTest.delta.to_json(None)
+        )
+        self.assertEqual(
+            "0:01:40",
+            IsoTimeTest.delta.to_json(100.0)
+        )
+
+        with self.assertRaises(ValueError) as cm:
+            IsoTimeTest.delta.to_json(datetime.timedelta(seconds=90000))
+
+        self.assertEqual(
+            cm.exception.message,
+            "IsoTime max value is 23:59:59=86400.0 seconds, but 90000.0 seconds is passed"
+        )
+
+        with self.assertRaises(TypeError):
+            IsoTimeTest.delta.to_json("123")