diff --git a/stix2/common.py b/stix2/common.py
index 1ec8cca..f8e2981 100644
--- a/stix2/common.py
+++ b/stix2/common.py
@@ -19,10 +19,10 @@ COMMON_PROPERTIES = {
 
 class ExternalReference(_STIXBase):
     _properties = {
-        'source_name': Property(required=True),
-        'description': Property(),
-        'url': Property(),
-        'external_id': Property(),
+        'source_name': StringProperty(required=True),
+        'description': StringProperty(),
+        'url': StringProperty(),
+        'external_id': StringProperty(),
     }
 
 
diff --git a/stix2/test/test_external_reference.py b/stix2/test/test_external_reference.py
index 5c39852..3b26675 100644
--- a/stix2/test/test_external_reference.py
+++ b/stix2/test/test_external_reference.py
@@ -1,6 +1,7 @@
 """Tests for stix.ExternalReference"""
 
 import pytest
+import re
 
 import stix2
 
@@ -34,7 +35,7 @@ def test_external_reference_capec():
     )
 
     assert str(ref) == CAPEC
-    assert repr(ref) == "ExternalReference(external_id='CAPEC-550', source_name='capec')"
+    assert re.match("ExternalReference\(external_id=u?'CAPEC-550', source_name=u?'capec'\)", repr(ref))
 
 
 CAPEC_URL = """{
@@ -101,7 +102,7 @@ def test_external_reference_offline():
     )
 
     assert str(ref) == OFFLINE
-    assert repr(ref) == "ExternalReference(description='Threat report', source_name='ACME Threat Intel')"
+    assert re.match("ExternalReference\(description=u?'Threat report', source_name=u?'ACME Threat Intel'\)", repr(ref))
     # Yikes! This works
     assert eval("stix2." + repr(ref)) == ref