diff --git a/lib/spack/spack/test/modules.py b/lib/spack/spack/test/modules.py
index c73badf8f24..0465c3e67fb 100644
--- a/lib/spack/spack/test/modules.py
+++ b/lib/spack/spack/test/modules.py
@@ -27,6 +27,7 @@
 
 import StringIO
 import spack.modules
+import unittest
 from spack.test.mock_packages_test import MockPackagesTest
 
 FILE_REGISTRY = collections.defaultdict(StringIO.StringIO)
@@ -100,6 +101,33 @@ def mock_open(filename, mode):
     }
 }
 
+class HelperFunctionsTests(unittest.TestCase):
+    def test_update_dictionary_extending_list(self):
+        target = {
+            'foo': {
+                'a': 1,
+                'b': 2,
+                'd': 4
+            },
+            'bar': [1, 2, 4],
+            'baz': 'foobar'
+        }
+        update = {
+            'foo': {
+                'c': 3,
+            },
+            'bar': [3],
+            'baz': 'foobaz',
+            'newkey': {
+                'd': 4
+            }
+        }
+        spack.modules.update_dictionary_extending_lists(target, update)
+        self.assertTrue(len(target) == 4)
+        self.assertTrue(len(target['foo']) == 4)
+        self.assertTrue(len(target['bar']) == 4)
+        self.assertEqual(target['baz'], 'foobaz')
+
 
 class TclTests(MockPackagesTest):
     def setUp(self):