diff --git a/test/test_cachefile_class.py b/test/test_cachefile_class.py
index fa07eac7..cefecac2 100644
--- a/test/test_cachefile_class.py
+++ b/test/test_cachefile_class.py
@@ -306,8 +306,9 @@ def assert_cache_unchanged(self, cache_file, cache_backup_file):
             assert cache.read() == backup.read()
 
     def test_read__cannot_modify(self, cache_file, full_cache_line, assert_cache_unchanged):
-        cache = Cache.read(cache_file)
-        cache.lines.append(**vars(full_cache_line))
+        cache = Cache.read(cache_file, read_only=True)
+        with pytest.raises(ValueError):
+            cache.lines.append(**vars(full_cache_line))
 
     def test_read__outdated(self):
         cache = Cache.read(XXL_CACHE_PATH)