diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index a14d9bd0c..9a700d39b 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -229,7 +229,7 @@ def format_event_for_client_v2_without_room_id(d):
 
 def serialize_event(e, time_now_ms, as_client_event=True,
                     event_format=format_event_for_client_v1,
-                    token_id=None, event_fields=None):
+                    token_id=None, only_event_fields=None):
     # FIXME(erikj): To handle the case of presence events and the like
     if not isinstance(e, EventBase):
         return e
@@ -258,7 +258,8 @@ def serialize_event(e, time_now_ms, as_client_event=True,
     if as_client_event:
         d = event_format(d)
 
-    if isinstance(event_fields, list):
-        d = only_fields(d, event_fields)
+    if (isinstance(only_event_fields, list) and
+            all(isinstance(f, basestring) for f in only_event_fields)):
+        d = only_fields(d, only_event_fields)
 
     return d
diff --git a/tests/events/test_utils.py b/tests/events/test_utils.py
index d415e4cb3..5b3326ce8 100644
--- a/tests/events/test_utils.py
+++ b/tests/events/test_utils.py
@@ -123,7 +123,7 @@ class PruneEventTestCase(unittest.TestCase):
 class SerializeEventTestCase(unittest.TestCase):
 
     def serialize(self, ev, fields):
-        return serialize_event(ev, 1924354, event_fields=fields)
+        return serialize_event(ev, 1479807801915, only_event_fields=fields)
 
     def test_event_fields_works_with_keys(self):
         self.assertEquals(
@@ -235,3 +235,58 @@ class SerializeEventTestCase(unittest.TestCase):
             ),
             {}
         )
+
+    def test_event_fields_nops_with_array_keys(self):
+        self.assertEquals(
+            self.serialize(
+                MockEvent(
+                    sender="@alice:localhost",
+                    room_id="!foo:bar",
+                    content={
+                        "foo": ["I", "am", "an", "array"],
+                    },
+                ),
+                ["content.foo.1"]
+            ),
+            {}
+        )
+
+    def test_event_fields_all_fields_if_empty(self):
+        self.assertEquals(
+            self.serialize(
+                MockEvent(
+                    room_id="!foo:bar",
+                    content={
+                        "foo": "bar",
+                    },
+                ),
+                []
+            ),
+            {
+                "room_id": "!foo:bar",
+                "content": {
+                    "foo": "bar",
+                },
+                "unsigned": {}
+            }
+        )
+
+    def test_event_fields_fail_if_fields_not_str(self):
+        self.assertEquals(
+            self.serialize(
+                MockEvent(
+                    room_id="!foo:bar",
+                    content={
+                        "foo": "bar",
+                    },
+                ),
+                ["room_id", 4]
+            ),
+            {
+                "room_id": "!foo:bar",
+                "content": {
+                    "foo": "bar",
+                },
+                "unsigned": {}
+            }
+        )