diff --git a/lib/spack/spack/operating_systems/mac_os.py b/lib/spack/spack/operating_systems/mac_os.py
index 3e5ab9b2e93..dafb5c1d41c 100644
--- a/lib/spack/spack/operating_systems/mac_os.py
+++ b/lib/spack/spack/operating_systems/mac_os.py
@@ -22,7 +22,7 @@ def __init__(self):
                         "10.11": "elcapitan",
                         "10.12": "sierra"}
 
-        mac_ver = py_platform.mac_ver()[0][:-2]
+        mac_ver = '.'.join(py_platform.mac_ver()[0].split('.')[:2])
         name = mac_releases.get(mac_ver, "macos")
         super(MacOs, self).__init__(name, mac_ver)