diff --git a/stix2/core.py b/stix2/core.py
index 96ce2ec..a82c149 100644
--- a/stix2/core.py
+++ b/stix2/core.py
@@ -293,12 +293,13 @@ def _collect_stix2_mappings():
         prefix = str(top_level_module.__name__) + '.'
 
         for module_loader, name, is_pkg in pkgutil.walk_packages(path=path, prefix=prefix):
+            ver = name.split('.')[1]
             if re.match(r'^stix2\.v2[0-9]$', name) and is_pkg:
                 mod = importlib.import_module(name, str(top_level_module.__name__))
-                STIX2_OBJ_MAPS[name.split('.')[1]] = {}
-                STIX2_OBJ_MAPS[name.split('.')[1]]['objects'] = mod.OBJ_MAP
-                STIX2_OBJ_MAPS[name.split('.')[1]]['observables'] = mod.OBJ_MAP_OBSERVABLE
-                STIX2_OBJ_MAPS[name.split('.')[1]]['observable-extensions'] = mod.EXT_MAP
+                STIX2_OBJ_MAPS[ver] = {}
+                STIX2_OBJ_MAPS[ver]['objects'] = mod.OBJ_MAP
+                STIX2_OBJ_MAPS[ver]['observables'] = mod.OBJ_MAP_OBSERVABLE
+                STIX2_OBJ_MAPS[ver]['observable-extensions'] = mod.EXT_MAP
             elif re.match(r'^stix2\.v2[0-9]\.common$', name) and is_pkg is False:
                 mod = importlib.import_module(name, str(top_level_module.__name__))
-                STIX2_OBJ_MAPS[name.split('.')[1]]['markings'] = mod.OBJ_MAP_MARKING
+                STIX2_OBJ_MAPS[ver]['markings'] = mod.OBJ_MAP_MARKING
diff --git a/stix2/v21/common.py b/stix2/v21/common.py
index 87c7225..0aded3b 100644
--- a/stix2/v21/common.py
+++ b/stix2/v21/common.py
@@ -98,7 +98,7 @@ class TLPMarking(_STIXBase):
 
     _type = 'tlp'
     _properties = OrderedDict([
-        ('tlp', Property(required=True)),
+        ('tlp', StringProperty(required=True)),
     ])