diff --git a/graphs_trees/bst_validate/bst_validate_challenge.ipynb b/graphs_trees/bst_validate/bst_validate_challenge.ipynb
index 270b384..e38dce6 100644
--- a/graphs_trees/bst_validate/bst_validate_challenge.ipynb
+++ b/graphs_trees/bst_validate/bst_validate_challenge.ipynb
@@ -36,7 +36,11 @@
     "\n",
     "* Can the tree have duplicates?\n",
     "    * Yes\n",
+    "* If this is called on a None input, should we raise an exception?\n",
+    "    * Yes\n",
     "* Can we assume we already have a Node class?\n",
+    "    * Yes\n",
+    "* Can we assume this fits in memory?\n",
     "    * Yes"
    ]
   },
@@ -128,10 +132,15 @@
    "source": [
     "# %load test_bst_validate.py\n",
     "from nose.tools import assert_equal\n",
+    "from nose.tools import raises\n",
     "\n",
     "\n",
     "class TestBstValidate(object):\n",
     "\n",
+    "    @raises(Exception)\n",
+    "    def test_bst_validate_empty(self):\n",
+    "        validate_bst(None)\n",
+    "\n",
     "    def test_bst_validate(self):\n",
     "        node = Node(5)\n",
     "        insert(node, 8)\n",
@@ -149,11 +158,13 @@
     "        root.right = right\n",
     "        root.left.right = invalid\n",
     "        assert_equal(validate_bst(root), False)\n",
+    "\n",
     "        print('Success: test_bst_validate')\n",
     "\n",
     "\n",
     "def main():\n",
     "    test = TestBstValidate()\n",
+    "    test.test_bst_validate_empty()\n",
     "    test.test_bst_validate()\n",
     "\n",
     "\n",
@@ -173,21 +184,21 @@
  ],
  "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/graphs_trees/bst_validate/bst_validate_solution.ipynb b/graphs_trees/bst_validate/bst_validate_solution.ipynb
index 1ff8045..2195a5e 100644
--- a/graphs_trees/bst_validate/bst_validate_solution.ipynb
+++ b/graphs_trees/bst_validate/bst_validate_solution.ipynb
@@ -35,7 +35,11 @@
     "\n",
     "* Can the tree have duplicates?\n",
     "    * Yes\n",
+    "* If this is called on a None input, should we raise an exception?\n",
+    "    * Yes\n",
     "* Can we assume we already have a Node class?\n",
+    "    * Yes\n",
+    "* Can we assume this fits in memory?\n",
     "    * Yes"
    ]
   },
@@ -45,6 +49,8 @@
    "source": [
     "## Test Cases\n",
     "\n",
+    "None -> exception\n",
+    "\n",
     "<pre>\n",
     "Valid:\n",
     "      5\n",
@@ -70,7 +76,7 @@
     "\n",
     "We'll use a recursive solution that valides left <= current < right, passing down the min and max values as we do a depth-first traversal.\n",
     "\n",
-    "* If the node is None, return False\n",
+    "* If the node is None, return True\n",
     "* If min is set and the node's value <= min, return False\n",
     "* if max is set and the node's value > max, return False\n",
     "* Recursively call the validate function on node.left, updating max\n",
@@ -108,19 +114,21 @@
    "outputs": [],
    "source": [
     "def validate_bst(node):\n",
-    "    return __validate_bst__(node, None, None)\n",
+    "    if node is None:\n",
+    "        raise Exception('No root node')\n",
+    "    return _validate_bst(node, None, None)\n",
     "\n",
     "\n",
-    "def __validate_bst__(node, mininum, maximum):\n",
+    "def _validate_bst(node, mininum, maximum):\n",
     "    if node is None:\n",
     "        return True\n",
     "    if mininum is not None and node.data <= mininum:\n",
     "        return False\n",
     "    if maximum is not None and node.data > maximum:\n",
     "        return False\n",
-    "    if not __validate_bst__(node.left, mininum, node.data):\n",
+    "    if not _validate_bst(node.left, mininum, node.data):\n",
     "        return False\n",
-    "    if not __validate_bst__(node.right, node.data, maximum):\n",
+    "    if not _validate_bst(node.right, node.data, maximum):\n",
     "        return False\n",
     "    return True"
    ]
@@ -150,10 +158,15 @@
    "source": [
     "%%writefile test_bst_validate.py\n",
     "from nose.tools import assert_equal\n",
+    "from nose.tools import raises\n",
     "\n",
     "\n",
     "class TestBstValidate(object):\n",
     "\n",
+    "    @raises(Exception)\n",
+    "    def test_bst_validate_empty(self):\n",
+    "        validate_bst(None)\n",
+    "\n",
     "    def test_bst_validate(self):\n",
     "        node = Node(5)\n",
     "        insert(node, 8)\n",
@@ -177,6 +190,7 @@
     "\n",
     "def main():\n",
     "    test = TestBstValidate()\n",
+    "    test.test_bst_validate_empty()\n",
     "    test.test_bst_validate()\n",
     "\n",
     "\n",
@@ -206,21 +220,21 @@
  ],
  "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/graphs_trees/bst_validate/test_bst_validate.py b/graphs_trees/bst_validate/test_bst_validate.py
index 059baed..815a03e 100644
--- a/graphs_trees/bst_validate/test_bst_validate.py
+++ b/graphs_trees/bst_validate/test_bst_validate.py
@@ -1,8 +1,13 @@
 from nose.tools import assert_equal
+from nose.tools import raises
 
 
 class TestBstValidate(object):
 
+    @raises(Exception)
+    def test_bst_validate_empty(self):
+        validate_bst(None)
+
     def test_bst_validate(self):
         node = Node(5)
         insert(node, 8)
@@ -26,6 +31,7 @@ class TestBstValidate(object):
 
 def main():
     test = TestBstValidate()
+    test.test_bst_validate_empty()
     test.test_bst_validate()