diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index 4febd98f4333..a14d9bd0ca9b 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -16,6 +16,8 @@
 from synapse.api.constants import EventTypes
 from . import EventBase
 
+from frozendict import frozendict
+
 import re
 
 # Split strings on "." but not "\." This uses a negative lookbehind assertion for '\'
@@ -130,7 +132,7 @@ def _copy_field(src, dst, field):
     key_to_move = field.pop(-1)
     sub_dict = src
     for sub_field in field:  # e.g. sub_field => "content"
-        if sub_field in sub_dict and type(sub_dict[sub_field]) == dict:
+        if sub_field in sub_dict and type(sub_dict[sub_field]) == frozendict:
             sub_dict = sub_dict[sub_field]
         else:
             return
diff --git a/tests/events/test_utils.py b/tests/events/test_utils.py
index b9f55d174dad..7136cca7c2f3 100644
--- a/tests/events/test_utils.py
+++ b/tests/events/test_utils.py
@@ -17,7 +17,11 @@
 from .. import unittest
 
 from synapse.events import FrozenEvent
-from synapse.events.utils import prune_event
+from synapse.events.utils import prune_event, serialize_event
+
+
+def MockEvent(**kwargs):
+    return FrozenEvent(kwargs)
 
 
 class PruneEventTestCase(unittest.TestCase):
@@ -118,11 +122,41 @@ def test_content(self):
 
 class SerializeEventTestCase(unittest.TestCase):
 
+    def serialize(self, ev, fields):
+        return serialize_event(ev, 1924354, event_fields=fields)
+
     def test_event_fields_works_with_keys(self):
-        pass
+        self.assertEquals(
+            self.serialize(
+                MockEvent(
+                    sender="@alice:localhost",
+                    room_id="!foo:bar"
+                ),
+                ["room_id"]
+            ),
+            {
+                "room_id": "!foo:bar",
+            }
+        )
 
     def test_event_fields_works_with_nested_keys(self):
-        pass
+        self.assertEquals(
+            self.serialize(
+                MockEvent(
+                    sender="@alice:localhost",
+                    room_id="!foo:bar",
+                    content={
+                        "body": "A message",
+                    },
+                ),
+                ["content.body"]
+            ),
+            {
+                "content": {
+                    "body": "A message",
+                }
+            }
+        )
 
     def test_event_fields_works_with_dot_keys(self):
         pass