diff --git a/tools/run_tests/run_microbenchmark.py b/tools/run_tests/run_microbenchmark.py
index f72ec72dcedf755d36fe82cb388dc22ef57b5ccd..c1c8a488ae3acf414ba88367fdc58dd5c6063fd9 100755
--- a/tools/run_tests/run_microbenchmark.py
+++ b/tools/run_tests/run_microbenchmark.py
@@ -235,7 +235,7 @@ for bm_name in args.benchmarks:
 if args.diff_perf:
   for bm_name in args.benchmarks:
     run_summary(bm_name, 'opt', '%s.new' % bm_name)
-  where_am_i = subprocess.check_call(['git', 'rev-parse', '--abbrev-ref', 'HEAD'])
+  where_am_i = subprocess.check_output(['git', 'rev-parse', '--abbrev-ref', 'HEAD'])
   subprocess.check_call(['git', 'checkout', args.diff_perf])
   comparables = []
   subprocess.check_call(['make', 'clean'])