diff --git a/tests/test_arbitrary_package_attack.py b/tests/test_arbitrary_package_attack.py
index 99cc2fb0..e6f7810a 100755
--- a/tests/test_arbitrary_package_attack.py
+++ b/tests/test_arbitrary_package_attack.py
@@ -266,7 +266,8 @@ def test_with_tuf_and_metadata_tampering(self):
     tuf.formats.check_signable_object_format(metadata)
 
     with open(metadata_path, 'wb') as file_object:
-      json.dumps(metadata, file_object, indent=1, sort_keys=True).encode('utf-8')
+      file_object.write(json.dumps(metadata, indent=1,
+          separators=(',', ': '), sort_keys=True).encode('utf-8'))
 
     # Verify that the malicious 'targets.json' is not downloaded.  Perform
     # a refresh of top-level metadata to demonstrate that the malicious