diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 09f3ab8..b199461 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -1547,7 +1547,7 @@
                 wanted_xcode_version
             ),
             "Bazel CI automatically picked a fallback version: {}.".format(xcode_version),
-            "Available versions are: {}.".format(supported_versions),
+            "Available versions are: {}.".format(", ".join(supported_versions)),
         ]
         execute_command(
             [