diff --git a/setup/unix-ci.py b/setup/unix-ci.py
index d569306371..36a49636fc 100644
--- a/setup/unix-ci.py
+++ b/setup/unix-ci.py
@@ -78,7 +78,7 @@ def download_and_decompress(url, dest, compression=None):
 def run_python(*args):
     python = os.path.expandvars('$SW/bin/python')
     if ismacos:
-        python += 2
+        python += '2'
     if len(args) == 1:
         args = shlex.split(args[0])
     args = [python] + list(args)