diff --git a/tests/ert/unit_tests/gui/simulation/test_run_dialog.py b/tests/ert/unit_tests/gui/simulation/test_run_dialog.py
index 52578a8b910..6548032e3a5 100644
--- a/tests/ert/unit_tests/gui/simulation/test_run_dialog.py
+++ b/tests/ert/unit_tests/gui/simulation/test_run_dialog.py
@@ -1,4 +1,5 @@
 import os
+from pathlib import Path
 from queue import SimpleQueue
 from unittest.mock import MagicMock, Mock, patch
 
@@ -17,7 +18,7 @@
 )
 
 import ert
-from ert.config import ErtConfig
+from ert.config import DesignMatrix, ErtConfig
 from ert.ensemble_evaluator import state
 from ert.ensemble_evaluator.event import (
     EndEvent,
@@ -720,14 +721,15 @@ def test_that_stdout_and_stderr_buttons_react_to_file_content(
 def test_that_design_matrix_show_parameters_button(
     design_matrix_entry, qtbot: QtBot, storage
 ):
-    with open("design_matrix.xlsx", "w", encoding="utf-8"):
+    xls_filename = "design_matrix.xls"
+    with open(f"{xls_filename}", "w", encoding="utf-8"):
         pass
     config_file = "minimal_config.ert"
     with open(config_file, "w", encoding="utf-8") as f:
         f.write("NUM_REALIZATIONS 1")
         if design_matrix_entry:
             f.write(
-                "\nDESIGN_MATRIX design_matrix.xlsx DESIGN_SHEET:DesignSheet01 DEFAULT_SHEET:DefaultValues"
+                f"\nDESIGN_MATRIX {xls_filename} DESIGN_SHEET:DesignSheet01 DEFAULT_SHEET:DefaultValues"
             )
 
     args_mock = Mock()
@@ -754,8 +756,7 @@ def test_that_design_matrix_show_parameters_button(
         assert show_dm_parameters.isEnabled() == design_matrix_entry
         if show_dm_parameters.isEnabled():
             design_matrix_df = pd.DataFrame({"a": [1, 2, 3], "b": [3, 4, 5]})
-            filename = "design_matrix.xlsl"
             with patch.object(
-                ert.config.DesignMatrix, "design_matrix_df", design_matrix_df
-            ), patch.object(ert.config.DesignMatrix, "xlsl_filename", filename):
+                DesignMatrix, "design_matrix_df", design_matrix_df
+            ), patch.object(DesignMatrix, "xls_filename", Path(xls_filename)):
                 qtbot.mouseClick(show_dm_parameters, Qt.LeftButton)