diff --git a/stix2/datastore/memory.py b/stix2/datastore/memory.py
index 7d5fb1c..a465116 100644
--- a/stix2/datastore/memory.py
+++ b/stix2/datastore/memory.py
@@ -51,14 +51,14 @@ def _add(store, stix_data, allow_custom=True, version=None):
         # Map ID directly to the object, if it is a marking.  Otherwise,
         # map to a family, so we can track multiple versions.
         if _is_marking(stix_obj):
-            store._data[stix_obj.id] = stix_obj
+            store._data[stix_obj["id"]] = stix_obj
 
         else:
-            if stix_obj.id in store._data:
-                obj_family = store._data[stix_obj.id]
+            if stix_obj["id"] in store._data:
+                obj_family = store._data[stix_obj["id"]]
             else:
                 obj_family = _ObjectFamily()
-                store._data[stix_obj.id] = obj_family
+                store._data[stix_obj["id"]] = obj_family
 
             obj_family.add(stix_obj)
 
@@ -71,8 +71,8 @@ def _is_marking(obj_or_id):
     :return: True if a marking definition, False otherwise.
     """
 
-    if isinstance(obj_or_id, _STIXBase):
-        id_ = obj_or_id.id
+    if isinstance(obj_or_id, (_STIXBase, dict)):
+        id_ = obj_or_id["id"]
     else:
         id_ = obj_or_id
 
@@ -91,15 +91,15 @@ class _ObjectFamily(object):
         self.latest_version = None
 
     def add(self, obj):
-        self.all_versions[obj.modified] = obj
+        self.all_versions[obj["modified"]] = obj
         if self.latest_version is None or \
-                obj.modified > self.latest_version.modified:
+                obj["modified"] > self.latest_version["modified"]:
             self.latest_version = obj
 
     def __str__(self):
         return "<<{}; latest={}>>".format(
             self.all_versions,
-            self.latest_version.modified,
+            self.latest_version["modified"],
         )
 
     def __repr__(self):
@@ -199,7 +199,7 @@ class MemorySink(DataSink):
         _add(self, stix_data, self.allow_custom)
     add.__doc__ = _add.__doc__
 
-    def save_to_file(self, path, encoding='utf-8'):
+    def save_to_file(self, path, encoding="utf-8"):
         path = os.path.abspath(path)
 
         all_objs = list(itertools.chain.from_iterable(
@@ -208,7 +208,7 @@ class MemorySink(DataSink):
             for value in self._data.values()
         ))
 
-        if any('spec_version' in x for x in all_objs):
+        if any("spec_version" in x for x in all_objs):
             bundle = v21.Bundle(all_objs, allow_custom=self.allow_custom)
         else:
             bundle = v20.Bundle(all_objs, allow_custom=self.allow_custom)
@@ -218,9 +218,9 @@ class MemorySink(DataSink):
 
         # if the user only provided a directory, use the bundle id for filename
         if os.path.isdir(path):
-            path = os.path.join(path, bundle['id'] + '.json')
+            path = os.path.join(path, bundle["id"] + ".json")
 
-        with io.open(path, 'w', encoding=encoding) as f:
+        with io.open(path, "w", encoding=encoding) as f:
             bundle = bundle.serialize(pretty=True, encoding=encoding, ensure_ascii=False)
             f.write(bundle)
     save_to_file.__doc__ = MemoryStore.save_to_file.__doc__