diff --git a/stix2/base.py b/stix2/base.py
index 5307393..b0cf6ff 100644
--- a/stix2/base.py
+++ b/stix2/base.py
@@ -40,7 +40,14 @@ class _STIXBase(collections.Mapping):
     """Base class for STIX object types"""
 
     def object_properties(self):
-        return list(self._properties.keys())
+        props = set(self._properties.keys())
+        custom_props = list(set(self._inner.keys()) - props)
+        custom_props.sort()
+
+        all_properties = list(self._properties.keys())
+        all_properties.extend(custom_props)  # Any custom properties to the bottom
+
+        return all_properties
 
     def _check_property(self, prop_name, prop, kwargs):
         if prop_name not in kwargs:
diff --git a/stix2/test/test_custom.py b/stix2/test/test_custom.py
index 48529b9..c5726b8 100644
--- a/stix2/test/test_custom.py
+++ b/stix2/test/test_custom.py
@@ -91,6 +91,7 @@ def test_custom_property_in_bundled_object():
     bundle = stix2.Bundle(identity, allow_custom=True)
 
     assert bundle.objects[0].x_foo == "bar"
+    assert '"x_foo": "bar"' in str(bundle)
 
 
 @stix2.sdo.CustomObject('x-new-type', [