diff --git a/spinedb_api/mapped_items.py b/spinedb_api/mapped_items.py
index 7348dcae..5d20cb60 100644
--- a/spinedb_api/mapped_items.py
+++ b/spinedb_api/mapped_items.py
@@ -822,7 +822,7 @@ def __init__(self, db_map, **kwargs):
         super().__init__(db_map, **kwargs)
         if "parameter_type_list" in kwargs:
             self._init_type_list = kwargs["parameter_type_list"]
-            if len(set(self._init_type_list)) != len(self._init_type_list):
+            if self._init_type_list is not None and len(set(self._init_type_list)) != len(self._init_type_list):
                 raise SpineDBAPIError("entries in parameter_type_list must be unique")
         else:
             self._init_type_list = None
diff --git a/tests/test_parameter_definitions.py b/tests/test_parameter_definitions.py
new file mode 100644
index 00000000..bc5c363b
--- /dev/null
+++ b/tests/test_parameter_definitions.py
@@ -0,0 +1,17 @@
+######################################################################################################################
+# Copyright (C) 2017-2022 Spine project consortium
+# Copyright Spine Database API contributors
+# This file is part of Spine Database API.
+# Spine Database API is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser
+# General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your
+# option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
+# without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General
+# Public License for more details. You should have received a copy of the GNU Lesser General Public License along with
+# this program. If not, see <http://www.gnu.org/licenses/>.
+######################################################################################################################
+def test_add_or_update_when_setting_parameter_type_list_to_none(db_map):
+    db_map.add_entity_class(name="Gadget")
+    definition = db_map.add_or_update_parameter_definition(
+        entity_class_name="Gadget", name="X", parameter_type_list=None
+    )
+    assert definition["parameter_type_list"] == ()