diff --git a/common/lib/xmodule/xmodule/combined_open_ended_module.py b/common/lib/xmodule/xmodule/combined_open_ended_module.py
index e01ae49149..256f96a3a7 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")
@@ -226,12 +226,10 @@ class CombinedOpenEndedFields(object):
     )
     due = Date(
         help="Date that this problem is due by",
-        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
     )
     version = VersionInteger(help="Current version number", default=DEFAULT_VERSION, scope=Scope.settings)
diff --git a/common/lib/xmodule/xmodule/peer_grading_module.py b/common/lib/xmodule/xmodule/peer_grading_module.py
index c29b6a8069..aa742f6a0b 100644
--- a/common/lib/xmodule/xmodule/peer_grading_module.py
+++ b/common/lib/xmodule/xmodule/peer_grading_module.py
@@ -46,7 +46,6 @@ class PeerGradingFields(object):
     )
     due = Date(
         help="Due date that should be displayed.",
-        default=None,
         scope=Scope.settings)
     graceperiod = Timedelta(
         help="Amount of grace to give on the due date.",
@@ -525,10 +524,10 @@ class PeerGradingModule(PeerGradingFields, XModule):
         good_problem_list = []
         for problem in problem_list:
             problem_location = problem['location']
-			try:
-            	descriptor = _find_corresponding_module_for_location(problem_location)
-			except:
-				continue
+            try:
+                descriptor = _find_corresponding_module_for_location(problem_location)
+            except:
+                continue
             if descriptor:
                 problem['due'] = descriptor.lms.due
                 grace_period = descriptor.lms.graceperiod
diff --git a/lms/djangoapps/courseware/model_data.py b/lms/djangoapps/courseware/model_data.py
index 44be16e441..b72c0e90c6 100644
--- a/lms/djangoapps/courseware/model_data.py
+++ b/lms/djangoapps/courseware/model_data.py
@@ -353,7 +353,8 @@ class LmsKeyValueStore(KeyValueStore):
         for field in kv_dict:
             # Check field for validity
             if field.field_name in self._descriptor_model_data:
-                raise InvalidWriteError("Not allowed to overwrite descriptor model data", field.field_name)
+                if field.field_name not in ["due","graceperiod"]:
+                    raise InvalidWriteError("Not allowed to overwrite descriptor model data", field.field_name)
 
             if field.scope not in self._allowed_scopes:
                 raise InvalidScopeError(field.scope)