Add test to ensure fix. Fixes #248

stix2/test/v20/test_malware.py

@@ -35,6 +35,22 @@ def test_malware_with_all_required_properties():
     assert str(mal) == EXPECTED_MALWARE
 
 
+def test_malware_with_empty_optional_field():
+    now = dt.datetime(2016, 5, 12, 8, 17, 27, tzinfo=pytz.utc)
+
+    mal = stix2.v20.Malware(
+        type="malware",
+        id=MALWARE_ID,
+        created=now,
+        modified=now,
+        labels=["ransomware"],
+        name="Cryptolocker",
+        external_references=[],
+    )
+
+    assert str(mal) == EXPECTED_MALWARE
+
+
 def test_malware_autogenerated_properties(malware):
     assert malware.type == 'malware'
     assert malware.id == 'malware--00000000-0000-4000-8000-000000000001'