diff --git a/lib/accesslib.php b/lib/accesslib.php
index cf029043ac1..4c81e300672 100644
--- a/lib/accesslib.php
+++ b/lib/accesslib.php
@@ -5779,6 +5779,13 @@ class context_helper extends context {
     protected function __construct() {
     }
 
+    /**
+     * Reset internal context levels array.
+     */
+    public static function reset_levels() {
+        self::$alllevels = null;
+    }
+
     /**
      * Initialise context levels, call before using self::$alllevels.
      */
@@ -5801,8 +5808,17 @@ class context_helper extends context {
             return;
         }
 
+        $levels = $CFG->custom_context_classes;
+        if (!is_array($levels)) {
+            $levels = @unserialize($levels);
+        }
+        if (!is_array($levels)) {
+            debugging('Invalid $CFG->custom_context_classes detected, value ignored.', DEBUG_DEVELOPER);
+            return;
+        }
+
         // Unsupported custom levels, use with care!!!
-        foreach ($CFG->custom_context_classes as $level => $classname) {
+        foreach ($levels as $level => $classname) {
             self::$alllevels[$level] = $classname;
         }
         ksort(self::$alllevels);
diff --git a/lib/tests/customcontext_test.php b/lib/tests/customcontext_test.php
new file mode 100644
index 00000000000..eaa2f3c888e
--- /dev/null
+++ b/lib/tests/customcontext_test.php
@@ -0,0 +1,138 @@
+<?php
+// This file is part of Moodle - http://moodle.org/
+//
+// Moodle is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// Moodle is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.
+
+/**
+ * Code quality unit tests that are fast enough to run each time.
+ *
+ * @package    core
+ * @category   phpunit
+ * @copyright  (C) 2013 onwards Remote Learner.net Inc (http://www.remote-learner.net)
+ * @author     Brent Boghosian (brent.boghosian@remote-learner.net)
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+defined('MOODLE_INTERNAL') || die();
+
+/**
+ * Bogus custom context class for testing
+ */
+class context_bogus1 extends context {
+    /**
+     * Returns the most relevant URL for this context.
+     *
+     * @return moodle_url
+     */
+    public function get_url() {
+        global $ME;
+        return $ME;
+    }
+
+    /**
+     * Returns array of relevant context capability records.
+     *
+     * @return array
+     */
+    public function get_capabilities() {
+        return array();
+    }
+}
+
+/**
+ * Bogus custom context class for testing
+ */
+class context_bogus2 extends context {
+    /**
+     * Returns the most relevant URL for this context.
+     *
+     * @return moodle_url
+     */
+    public function get_url() {
+        global $ME;
+        return $ME;
+    }
+
+    /**
+     * Returns array of relevant context capability records.
+     *
+     * @return array
+     */
+    public function get_capabilities() {
+        return array();
+    }
+}
+
+/**
+ * Bogus custom context class for testing
+ */
+class context_bogus3 extends context {
+    /**
+     * Returns the most relevant URL for this context.
+     *
+     * @return moodle_url
+     */
+    public function get_url() {
+        global $ME;
+        return $ME;
+    }
+
+    /**
+     * Returns array of relevant context capability records.
+     *
+     * @return array
+     */
+    public function get_capabilities() {
+        return array();
+    }
+}
+
+class customcontext_testcase extends advanced_testcase {
+
+    /**
+     * Perform setup before every test. This tells Moodle's phpunit to reset the database after every test.
+     */
+    protected function setUp() {
+        parent::setUp();
+        $this->resetAfterTest(true);
+    }
+
+    /**
+     * Test case for custom context classes
+     */
+    public function test_customcontexts() {
+        global $CFG;
+        static $customcontexts = array(
+            11 => 'context_bogus1',
+            12 => 'context_bogus2',
+            13 => 'context_bogus3'
+        );
+
+        // save any existing custom contexts
+        $existingcustomcontexts = get_config(null, 'custom_context_classes');
+
+        set_config('custom_context_classes', serialize($customcontexts));
+        initialise_cfg();
+        context_helper::reset_levels();
+        $alllevels = context_helper::get_all_levels();
+        $this->assertEquals($alllevels[11], 'context_bogus1');
+        $this->assertEquals($alllevels[12], 'context_bogus2');
+        $this->assertEquals($alllevels[13], 'context_bogus3');
+
+        // clean-up & restore any custom contexts
+        set_config('custom_context_classes', ($existingcustomcontexts === false) ? null : $existingcustomcontexts);
+        initialise_cfg();
+        context_helper::reset_levels();
+    }
+}