diff --git a/lib/spack/spack/spec.py b/lib/spack/spack/spec.py
index 1300f35ca48..b9d9d3e0a46 100644
--- a/lib/spack/spack/spec.py
+++ b/lib/spack/spack/spec.py
@@ -438,8 +438,7 @@ def copy(self):
         return clone
 
     def _cmp_key(self):
-        return ''.join(str(key) + ' '.join(str(v) for v in value)
-                       for key, value in sorted(self.items()))
+        return tuple((k, tuple(v)) for k, v in sorted(self.iteritems()))
 
     def __str__(self):
         sorted_keys = filter(