diff --git a/rdfproxy/utils/mapper_utils.py b/rdfproxy/utils/mapper_utils.py
index 05764e4..affeedc 100644
--- a/rdfproxy/utils/mapper_utils.py
+++ b/rdfproxy/utils/mapper_utils.py
@@ -15,8 +15,13 @@ def _is_list_type(obj: type | None) -> bool:
     return _is_type(obj, list)
 
 
-def _is_scalar_type(t) -> bool:
-    return (not _is_list_type(t)) and (not issubclass(t, BaseModel))
+def _is_scalar_type(obj) -> bool:
+    if obj is None:
+        return True
+    elif args := get_args(obj):
+        return all(_is_scalar_type(o) for o in args)
+    else:
+        return (not _is_list_type(obj)) and (not issubclass(obj, BaseModel))
 
 
 def _is_list_basemodel_type(obj: type | None) -> bool: