diff --git a/tardis/conftest.py b/tardis/conftest.py
index 1d42ff9d45e..6c0de744eee 100644
--- a/tardis/conftest.py
+++ b/tardis/conftest.py
@@ -107,7 +107,7 @@ def tardis_ref_path():
 @pytest.yield_fixture(scope="session")
 def tardis_ref_data(tardis_ref_path, generate_reference):
     if generate_reference:
-        mode = 'a'
+        mode = 'w'
     else:
         mode = 'r'
     with pd.HDFStore(
diff --git a/tardis/plasma/tests/test_complete_plasmas.py b/tardis/plasma/tests/test_complete_plasmas.py
index 192d8b0c119..5f2ffeb1d4e 100644
--- a/tardis/plasma/tests/test_complete_plasmas.py
+++ b/tardis/plasma/tests/test_complete_plasmas.py
@@ -134,9 +134,16 @@ def config(self, request):
         return config
 
     @pytest.yield_fixture(scope="class")
-    def reference(self, reference_fpath):
-        with pd.HDFStore(reference_fpath) as hdf_file:
-            yield hdf_file
+    def reference(self, reference_fpath, generate_reference):
+        if generate_reference:
+            mode = 'w'
+        else:
+            mode = 'r'
+        with pd.HDFStore(
+                reference_fpath,
+                mode=mode
+                ) as store:
+            yield store
 
     @pytest.fixture(scope="class")
     def plasma(self, chianti_he_db_fpath, config, reference_fpath, reference):