diff --git a/stix2/base.py b/stix2/base.py
index 35360af..a44d879 100644
--- a/stix2/base.py
+++ b/stix2/base.py
@@ -23,13 +23,7 @@ class STIXJSONEncoder(json.JSONEncoder):
 
 
 def get_required_properties(properties):
-    for k, v in properties.items():
-        if isinstance(v, dict):
-            if v.get('required'):
-                yield k
-        else:  # This is a Property subclass
-            if v.required:
-                yield k
+    return (k for k, v in properties.items() if v.required)
 
 
 class _STIXBase(collections.Mapping):
diff --git a/stix2/test/test_base.py b/stix2/test/test_base.py
new file mode 100644
index 0000000..38a3e45
--- /dev/null
+++ b/stix2/test/test_base.py
@@ -0,0 +1,25 @@
+import datetime as dt
+import json
+
+import pytest
+import pytz
+
+from stix2.base import STIXJSONEncoder
+
+
+def test_encode_json_datetime():
+    now = dt.datetime(2017, 3, 22, 0, 0, 0, tzinfo=pytz.UTC)
+    test_dict = {'now': now}
+
+    expected = '{"now": "2017-03-22T00:00:00Z"}'
+    assert json.dumps(test_dict, cls=STIXJSONEncoder) == expected
+
+
+def test_encode_json_object():
+    obj = object()
+    test_dict = {'obj': obj}
+
+    with pytest.raises(TypeError) as excinfo:
+        json.dumps(test_dict, cls=STIXJSONEncoder)
+
+    assert str(excinfo.value) == "Object of type 'object' is not JSON serializable"