diff --git a/common/lib/xmodule/xmodule/combined_open_ended_module.py b/common/lib/xmodule/xmodule/combined_open_ended_module.py
index e01ae49149..74082df1ce 100644
--- a/common/lib/xmodule/xmodule/combined_open_ended_module.py
+++ b/common/lib/xmodule/xmodule/combined_open_ended_module.py
@@ -8,7 +8,7 @@ from .x_module import XModule
 from xblock.core import Integer, Scope, String, List, Float, Boolean
 from xmodule.open_ended_grading_classes.combined_open_ended_modulev1 import CombinedOpenEndedV1Module, CombinedOpenEndedV1Descriptor
 from collections import namedtuple
-from .fields import Date
+from .fields import Date, Timedelta
 import textwrap
 
 log = logging.getLogger("mitx.courseware")
@@ -229,7 +229,7 @@ class CombinedOpenEndedFields(object):
         default=None,
         scope=Scope.settings
     )
-    graceperiod = String(
+    graceperiod = Timedelta(
         help="Amount of time after the due date that submissions will be accepted",
         default=None,
         scope=Scope.settings
diff --git a/common/lib/xmodule/xmodule/fields.py b/common/lib/xmodule/xmodule/fields.py
index dc2f000286..b7094203c4 100644
--- a/common/lib/xmodule/xmodule/fields.py
+++ b/common/lib/xmodule/xmodule/fields.py
@@ -82,6 +82,9 @@ TIMEDELTA_REGEX = re.compile(r'^((?P<days>\d+?) day(?:s?))?(\s)?((?P<hours>\d+?)
 
 
 class Timedelta(ModelType):
+    # Timedeltas are immutable, see http://docs.python.org/2/library/datetime.html#available-types
+    MUTABLE = False
+
     def from_json(self, time_str):
         """
         time_str: A string with the following components: