diff --git a/lld/utils/benchmark.py b/lld/utils/benchmark.py
--- a/lld/utils/benchmark.py
+++ b/lld/utils/benchmark.py
@@ -119,7 +119,7 @@
     tests = [runBench(b) for b in benchmarks]
     end = datetime.datetime.utcnow().isoformat()
     ret = {
-        'format_version' : 2,
+        'format_version' : '2',
         'machine' : { 'name' : args.machine },
         'run' : {
             'end_time' : start,