diff --git a/tools/testrunner/local/progress.py b/tools/testrunner/local/progress.py
index dcdf754be3..84acebabe8 100644
--- a/tools/testrunner/local/progress.py
+++ b/tools/testrunner/local/progress.py
@@ -333,7 +333,7 @@ class JsonTestProgressIndicator(ProgressIndicator):
       "stderr": test.output.stderr,
       "exit_code": test.output.exit_code,
       "result": test.suite.GetOutcome(test),
-      "expected": test.outcomes or ["PASS"],
+      "expected": list(test.outcomes) or ["PASS"],
     })