diff --git a/src/borg/testsuite/hashindex.py b/src/borg/testsuite/hashindex.py
index d5b6149ce..ce285bcd3 100644
--- a/src/borg/testsuite/hashindex.py
+++ b/src/borg/testsuite/hashindex.py
@@ -258,9 +258,9 @@ class HashIndexSizeTestCase(BaseTestCase):
         idx = ChunkIndex()
         for i in range(1234):
             idx[H(i)] = i, i**2
-        with tempfile.NamedTemporaryFile() as file:
-            idx.write(file)
-            size = os.path.getsize(file.fileno())
+        with unopened_tempfile() as filepath:
+            idx.write(filepath)
+            size = os.path.getsize(filepath)
         assert idx.size() == size