diff --git a/stix2/base.py b/stix2/base.py
index dbe6461..a13cb98 100644
--- a/stix2/base.py
+++ b/stix2/base.py
@@ -23,6 +23,7 @@ from .utils import revoke as _revoke
 __all__ = ['STIXJSONEncoder', '_STIXBase']
 
 DEFAULT_ERROR = "{type} must have {property}='{expected}'."
+SCO_DET_ID_NAMESPACE = uuid.UUID("00abedb4-aa42-466c-9c01-fed23315a9b7")
 
 
 class STIXJSONEncoder(json.JSONEncoder):
@@ -368,7 +369,6 @@ class _Observable(_STIXBase):
 
     def _generate_id(self, kwargs):
         required_prefix = self._type + "--"
-        namespace = uuid.UUID("00abedb4-aa42-466c-9c01-fed23315a9b7")
 
         properties_to_use = self._id_contributing_properties
         if properties_to_use:
@@ -396,9 +396,9 @@ class _Observable(_STIXBase):
 
                 # try/except here to enable python 2 compatibility
                 try:
-                    return required_prefix + six.text_type(uuid.uuid5(namespace, data))
+                    return required_prefix + six.text_type(uuid.uuid5(SCO_DET_ID_NAMESPACE, data))
                 except UnicodeDecodeError:
-                    return required_prefix + six.text_type(uuid.uuid5(namespace, six.binary_type(data)))
+                    return required_prefix + six.text_type(uuid.uuid5(SCO_DET_ID_NAMESPACE, six.binary_type(data)))
 
         # We return None if there are no values specified for any of the id-contributing-properties
         return None
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index 9fffeed..0d27bb4 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -50,7 +50,7 @@ class Artifact(_Observable):
 
     def _check_object_constraints(self):
         super(Artifact, self)._check_object_constraints()
-        self._check_mutually_exclusive_properties(['payload_bin', 'url'], at_least_one=False)
+        self._check_mutually_exclusive_properties(['payload_bin', 'url'])
         self._check_properties_dependency(['hashes'], ['url'])