diff --git a/src/borg/testsuite/hashindex.py b/src/borg/testsuite/hashindex.py
index 89a793a97..fef6c5dd4 100644
--- a/src/borg/testsuite/hashindex.py
+++ b/src/borg/testsuite/hashindex.py
@@ -197,8 +197,8 @@ class HashIndexSizeTestCase(BaseTestCase):
         for i in range(1234):
             idx[H(i)] = i, i**2, i**3
         with tempfile.NamedTemporaryFile() as file:
-            idx.write(file.name)
-            size = os.path.getsize(file.name)
+            idx.write(file)
+            size = os.path.getsize(file.fileno())
         assert idx.size() == size