diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 6580f46..d3a651f 100644
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -361,7 +361,7 @@
   if not targets:
     return
   print("\n+++ Build")
-  num_jobs = multiprocessing.cpu_count()
+  num_jobs = str(multiprocessing.cpu_count())
   common_flags = ["--color=yes", "--keep_going", "--jobs=" + num_jobs]
   fail_if_nonzero(execute_command(
       [bazel_binary, "build"] + common_flags + flags + targets))
@@ -371,7 +371,7 @@
   if not targets:
     return 0
   print("\n+++ Test")
-  num_jobs = multiprocessing.cpu_count()
+  num_jobs = str(multiprocessing.cpu_count())
   common_flags = ["--color=yes", "--keep_going", "--jobs=" + num_jobs,
                   "--local_test_jobs=" + num_jobs, "--build_event_json_file=" + bep_file]
   return execute_command([bazel_binary, "test"] + common_flags + flags + targets)