diff --git a/jsonfield/fields.py b/jsonfield/fields.py
index 21a6e23..f5a6d02 100644
--- a/jsonfield/fields.py
+++ b/jsonfield/fields.py
@@ -21,6 +21,13 @@
 from .encoder import JSONEncoder
 
 
+def polymorphic_has_pk(obj):
+    if getattr(obj, 'polymorphic_primary_key_name', None):
+        if getattr(obj, obj.polymorphic_primary_key_name, None):
+            return True
+    return False
+
+
 class JSONFormFieldBase(object):
     def __init__(self, *args, **kwargs):
         self.load_kwargs = kwargs.pop('load_kwargs', {})
@@ -77,7 +84,9 @@ def pre_init(self, value, obj):
                 # Make sure the primary key actually exists on the object before
                 # checking if it's empty. This is a special case for South datamigrations
                 # see: https://github.com/bradjasper/django-jsonfield/issues/52
-                if getattr(obj, "pk", None) is not None:
+                # Check if its a polymorphic objects and has a primary key
+                # see: https://github.com/dmkoch/django-jsonfield/issues/101
+                if getattr(obj, "pk", None) is not None or polymorphic_has_pk(obj):
                     if isinstance(value, six.string_types):
                         try:
                             return json.loads(value, **self.load_kwargs)
diff --git a/jsonfield/tests.py b/jsonfield/tests.py
index 2d0b980..950cea9 100644
--- a/jsonfield/tests.py
+++ b/jsonfield/tests.py
@@ -390,3 +390,28 @@ def test_get_prep_value_can_return_none_if_null(self):
         self.assertDictEqual(value,
                              json.loads(json.loads(double_prepared_value)))
         self.assertIs(json_field_instance.get_prep_value(None), None)
+
+
+class JsonModelWithCustomPrimaryKey(models.Model):
+    my_key = models.PositiveIntegerField(primary_key=True)
+    json = JSONField()
+    default_json = JSONField(default={"check": 12})
+    complex_default_json = JSONField(default=[{"checkcheck": 1212}])
+    empty_default = JSONField(default={})
+
+
+class JSONFieldWithCustomPrimaryKeyTest(TestCase):
+    """Tests objects with custom primary key"""
+
+    json_model = JsonModelWithCustomPrimaryKey
+
+    def test_json_field_create(self):
+        """Test saving a JSON object in our JSONField on an object with custom pk"""
+        json_obj = {
+            "item_1": "this is a json blah",
+            "blergh": "hey, hey, hey"}
+
+        obj = self.json_model.objects.create(json=json_obj, my_key=1)
+        new_obj = self.json_model.objects.get(pk=obj.pk)
+
+        self.assertEqual(new_obj.json, json_obj)