diff --git a/spinedb_api/arrow_value.py b/spinedb_api/arrow_value.py
index f0ac1704..7dae65e9 100644
--- a/spinedb_api/arrow_value.py
+++ b/spinedb_api/arrow_value.py
@@ -143,7 +143,7 @@ def crawl_map_uneven(
     typed_xs = []
     ys = []
     max_nested_depth = 0
-    index_names = root_index_names + [loaded_value.get("index_name", "x")]
+    index_names = root_index_names + [loaded_value.get("index_name", f"col_{depth}")]
     index_metadata = {}
     deepest_nested_index_names = []
     index_type = loaded_value["index_type"]
diff --git a/tests/test_arrow_value.py b/tests/test_arrow_value.py
index c0f8184c..ee758bfa 100644
--- a/tests/test_arrow_value.py
+++ b/tests/test_arrow_value.py
@@ -64,8 +64,8 @@ def test_empty_map(self):
         value, value_type = parameter_value.to_database(parameter_value.Map([], [], str))
         map_ = arrow_value.from_database(value, value_type)
         self.assertEqual(len(map_), 0)
-        self.assertEqual(map_.column_names, ["x", "value"])
-        self.assertEqual(map_.column("x").type, pyarrow.string())
+        self.assertEqual(map_.column_names, ["col_1", "value"])
+        self.assertEqual(map_.column("col_1").type, pyarrow.string())
         self.assertEqual(map_.column("value").type, pyarrow.null())
 
     def test_string_to_string_map_with_index_name(self):
@@ -141,7 +141,7 @@ def test_unevenly_nested_map_with_fixed_resolution_time_series(self):
         )
         map_ = arrow_value.from_database(value, value_type)
         self.assertEqual(len(map_), 6)
-        self.assertEqual(map_.column_names, ["main index", "nested index", "x", "t", "value"])
+        self.assertEqual(map_.column_names, ["main index", "nested index", "col_3", "t", "value"])
         self.assertEqual(map_.column("main index").type, pyarrow.string())
         self.assertEqual(
             map_.column("main index").to_pylist(),
@@ -149,7 +149,7 @@ def test_unevenly_nested_map_with_fixed_resolution_time_series(self):
         )
         self.assertEqual(map_.column("nested index").to_pylist(), [None, 11.0, "ts", "ts", "no ts", "key"])
         self.assertEqual(
-            map_.column("x").to_pylist(),
+            map_.column("col_3").to_pylist(),
             [
                 None,
                 None,
@@ -191,7 +191,7 @@ def test_unevenly_nested_map(self):
         )
         map_ = arrow_value.from_database(value, value_type)
         self.assertEqual(len(map_), 6)
-        self.assertEqual(map_.column_names, ["main index", "nested index", "x", "value"])
+        self.assertEqual(map_.column_names, ["main index", "nested index", "col_3", "value"])
         self.assertEqual(map_.column("main index").type, pyarrow.string())
         self.assertEqual(
             map_.column("main index").to_pylist(),
@@ -201,7 +201,7 @@ def test_unevenly_nested_map(self):
             map_.column("nested index").to_pylist(), [None, 11.0, "date time", "more date time", "non nested", "key"]
         )
         self.assertEqual(
-            map_.column("x").to_pylist(),
+            map_.column("col_3").to_pylist(),
             [None, None, "time of my life", datetime.datetime.fromisoformat("2024-02-26T17:45:00"), None, None],
         )
         self.assertEqual(
diff --git a/tests/test_dataframes.py b/tests/test_dataframes.py
index b84dfbbd..e48476fe 100644
--- a/tests/test_dataframes.py
+++ b/tests/test_dataframes.py
@@ -78,6 +78,33 @@ def test_simple_map_value(self):
             )
             self.assertTrue(dataframe.equals(expected))
 
+    def test_unnamed_indexes_are_named_as_col_x(self):
+        with DatabaseMapping("sqlite://", create=True) as db_map:
+            self._assert_success(db_map.add_entity_class_item(name="Object"))
+            self._assert_success(db_map.add_parameter_definition_item(name="y", entity_class_name="Object"))
+            self._assert_success(db_map.add_entity_item(name="fork", entity_class_name="Object"))
+            value_item = self._assert_success(
+                db_map.add_parameter_value_item(
+                    entity_class_name="Object",
+                    entity_byname=("fork",),
+                    parameter_definition_name="y",
+                    alternative_name="Base",
+                    parsed_value=Map(["A"], [1.1]),
+                )
+            )
+            dataframe = spine_df.to_dataframe(value_item)
+            expected = pd.DataFrame(
+                {
+                    "entity_class_name": pd.Series(["Object"], dtype="category"),
+                    "Object": pd.Series(["fork"], dtype="string"),
+                    "parameter_definition_name": pd.Series(["y"], dtype="category"),
+                    "alternative_name": pd.Series(["Base"], dtype="category"),
+                    "col_1": ["A"],
+                    "value": [1.1],
+                }
+            )
+            self.assertTrue(dataframe.equals(expected))
+
     def test_time_series_value(self):
         with DatabaseMapping("sqlite://", create=True) as db_map:
             self._assert_success(db_map.add_entity_class_item(name="Object"))