diff --git a/arrays_strings/permutation/permutation_challenge.ipynb b/arrays_strings/permutation/permutation_challenge.ipynb
index ce31923..05426fb 100644
--- a/arrays_strings/permutation/permutation_challenge.ipynb
+++ b/arrays_strings/permutation/permutation_challenge.ipynb
@@ -40,6 +40,10 @@
     "* Is whitespace important?\n",
     "    * Yes\n",
     "* Is this case sensitive?  'Nib', 'bin' is not a match?\n",
+    "    * Yes\n",
+    "* Can we use additional data structures?\n",
+    "    * Yes\n",
+    "* Can we assume this fits in memory?\n",
     "    * Yes"
    ]
   },
@@ -49,6 +53,7 @@
    "source": [
     "## Test Cases\n",
     "\n",
+    "* One or more None inputs -> False\n",
     "* One or more empty strings -> False\n",
     "* 'Nib', 'bin' -> False\n",
     "* 'act', 'cat' -> True\n",
@@ -115,6 +120,7 @@
     "class TestPermutation(object):\n",
     "\n",
     "    def test_permutation(self, func):\n",
+    "        assert_equal(func(None, 'foo'), False)\n",
     "        assert_equal(func('', 'foo'), False)\n",
     "        assert_equal(func('Nib', 'bin'), False)\n",
     "        assert_equal(func('act', 'cat'), True)\n",
@@ -149,21 +155,21 @@
  ],
  "metadata": {
   "kernelspec": {
-   "display_name": "Python 2",
+   "display_name": "Python 3",
    "language": "python",
-   "name": "python2"
+   "name": "python3"
   },
   "language_info": {
    "codemirror_mode": {
     "name": "ipython",
-    "version": 2
+    "version": 3
    },
    "file_extension": ".py",
    "mimetype": "text/x-python",
    "name": "python",
    "nbconvert_exporter": "python",
-   "pygments_lexer": "ipython2",
-   "version": "2.7.10"
+   "pygments_lexer": "ipython3",
+   "version": "3.5.0"
   }
  },
  "nbformat": 4,
diff --git a/arrays_strings/permutation/permutation_solution.ipynb b/arrays_strings/permutation/permutation_solution.ipynb
index 23be349..5348686 100644
--- a/arrays_strings/permutation/permutation_solution.ipynb
+++ b/arrays_strings/permutation/permutation_solution.ipynb
@@ -41,6 +41,10 @@
     "* Is whitespace important?\n",
     "    * Yes\n",
     "* Is this case sensitive?  'Nib', 'bin' is not a match?\n",
+    "    * Yes\n",
+    "* Can we use additional data structures?\n",
+    "    * Yes\n",
+    "* Can we assume this fits in memory?\n",
     "    * Yes"
    ]
   },
@@ -50,6 +54,7 @@
    "source": [
     "## Test Cases\n",
     "\n",
+    "* One or more None inputs -> False\n",
     "* One or more empty strings -> False\n",
     "* 'Nib', 'bin' -> False\n",
     "* 'act', 'cat' -> True\n",
@@ -91,6 +96,8 @@
    "outputs": [],
    "source": [
     "def permutations(str1, str2):\n",
+    "    if str1 is None or str2 is None:\n",
+    "        return False\n",
     "    return sorted(str1) == sorted(str2)"
    ]
   },
@@ -141,6 +148,8 @@
     "\n",
     "\n",
     "def permutations_alt(str1, str2):\n",
+    "    if str1 is None or str2 is None:\n",
+    "        return False\n",
     "    if len(str1) != len(str2):\n",
     "        return False\n",
     "    unique_counts1 = defaultdict(int)\n",
@@ -182,6 +191,7 @@
     "class TestPermutation(object):\n",
     "\n",
     "    def test_permutation(self, func):\n",
+    "        assert_equal(func(None, 'foo'), False)\n",
     "        assert_equal(func('', 'foo'), False)\n",
     "        assert_equal(func('Nib', 'bin'), False)\n",
     "        assert_equal(func('act', 'cat'), True)\n",
diff --git a/arrays_strings/permutation/test_permutation_solution.py b/arrays_strings/permutation/test_permutation_solution.py
index 9e14427..23929a5 100644
--- a/arrays_strings/permutation/test_permutation_solution.py
+++ b/arrays_strings/permutation/test_permutation_solution.py
@@ -4,6 +4,7 @@ from nose.tools import assert_equal
 class TestPermutation(object):
 
     def test_permutation(self, func):
+        assert_equal(func(None, 'foo'), False)
         assert_equal(func('', 'foo'), False)
         assert_equal(func('Nib', 'bin'), False)
         assert_equal(func('act', 'cat'), True)