diff --git a/rispy/config.py b/rispy/config.py
index a66e088..6a19e61 100644
--- a/rispy/config.py
+++ b/rispy/config.py
@@ -1,6 +1,6 @@
 """Define default mappings."""
 
-LIST_TYPE_TAGS = [
+LIST_TYPE_TAGS = {
     "A1",
     "A2",
     "A3",
@@ -9,7 +9,7 @@
     "KW",
     "N1",
     "UR",
-]
+}
 
 DELIMITED_TAG_MAPPING = {
     "UR": ";",
@@ -141,7 +141,7 @@
     "VIDEO": "Video recording",
 }
 
-WOK_LIST_TYPE_TAGS = [
+WOK_LIST_TYPE_TAGS = {
     "RI",
     "CR",
     "AF",
@@ -150,7 +150,7 @@
     "AU",
     "CA",
     "GP",
-]
+}
 
 WOK_TAG_KEY_MAPPING = {
     "FN": "file_name",
diff --git a/tests/test_writer.py b/tests/test_writer.py
index 809a965..8094c20 100644
--- a/tests/test_writer.py
+++ b/tests/test_writer.py
@@ -67,7 +67,7 @@ def test_dumps_multiple_unknown_tags_ris(tmp_path):
 def test_custom_list_tags():
     filepath = DATA_DIR / "example_custom_list_tags.ris"
     list_tags = deepcopy(rispy.LIST_TYPE_TAGS)
-    list_tags.append("SN")
+    list_tags.add("SN")
 
     expected = {
         "type_of_reference": "JOUR",