diff --git a/lib/spack/spack/test/environment.py b/lib/spack/spack/test/environment.py
index 97581ecb76a..f44282d6f39 100644
--- a/lib/spack/spack/test/environment.py
+++ b/lib/spack/spack/test/environment.py
@@ -62,4 +62,4 @@ def test_extend(self):
         copy_construct = EnvironmentModifications(env)
         self.assertEqual(len(copy_construct), 2)
         for x, y in zip(env, copy_construct):
-            self.assertIs(x, y)
+            assert x is y