diff --git a/webservice/rest/locallib.php b/webservice/rest/locallib.php
index 1ead1513413..f0bd340b166 100644
--- a/webservice/rest/locallib.php
+++ b/webservice/rest/locallib.php
@@ -224,7 +224,8 @@ class webservice_rest_server extends webservice_base_server {
         } else if ($desc instanceof external_single_structure) {
             $single = '<SINGLE>'."\n";
             foreach ($desc->keys as $key=>$subdesc) {
-                $single .= '<KEY name="'.$key.'">'.self::xmlize_result($returns[$key], $subdesc).'</KEY>'."\n";
+                $value = isset($returns[$key]) ? $returns[$key] : null;
+                $single .= '<KEY name="'.$key.'">'.self::xmlize_result($value, $subdesc).'</KEY>'."\n";
             }
             $single .= '</SINGLE>'."\n";
             return $single;
diff --git a/webservice/rest/tests/server_test.php b/webservice/rest/tests/server_test.php
new file mode 100644
index 00000000000..354fc6048f4
--- /dev/null
+++ b/webservice/rest/tests/server_test.php
@@ -0,0 +1,217 @@
+<?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/>.
+
+/**
+ * Rest server tests.
+ *
+ * @package    webservice_rest
+ * @copyright  2016 Frédéric Massart - FMCorz.net
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+
+defined('MOODLE_INTERNAL') || die();
+global $CFG;
+
+require_once($CFG->libdir . '/externallib.php');
+require_once($CFG->dirroot . '/webservice/rest/locallib.php');
+
+/**
+ * Rest server testcase.
+ *
+ * @package    webservice_rest
+ * @copyright  2016 Frédéric Massart - FMCorz.net
+ * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
+ */
+class webservice_rest_server_testcase extends advanced_testcase {
+
+    /**
+     * Data provider for test_xmlize.
+     * @return array
+     */
+    public function xmlize_provider() {
+        $data = [];
+        $data[] = [null, null, ''];
+        $data[] = [new external_value(PARAM_BOOL), false, "<VALUE>0</VALUE>\n"];
+        $data[] = [new external_value(PARAM_BOOL), true, "<VALUE>1</VALUE>\n"];
+        $data[] = [new external_value(PARAM_ALPHA), null, "<VALUE null=\"null\"/>\n"];
+        $data[] = [new external_value(PARAM_ALPHA), 'a', "<VALUE>a</VALUE>\n"];
+        $data[] = [new external_value(PARAM_INT), 123, "<VALUE>123</VALUE>\n"];
+        $data[] = [
+            new external_multiple_structure(new external_value(PARAM_INT)),
+            [1, 2, 3],
+            "<MULTIPLE>\n" .
+            "<VALUE>1</VALUE>\n" .
+            "<VALUE>2</VALUE>\n" .
+            "<VALUE>3</VALUE>\n" .
+            "</MULTIPLE>\n"
+        ];
+        $data[] = [ // Multiple structure with null value.
+            new external_multiple_structure(new external_value(PARAM_ALPHA)),
+            ['A', null, 'C'],
+            "<MULTIPLE>\n" .
+            "<VALUE>A</VALUE>\n" .
+            "<VALUE null=\"null\"/>\n" .
+            "<VALUE>C</VALUE>\n" .
+            "</MULTIPLE>\n"
+        ];
+        $data[] = [ // Multiple structure without values.
+            new external_multiple_structure(new external_value(PARAM_ALPHA)),
+            [],
+            "<MULTIPLE>\n" .
+            "</MULTIPLE>\n"
+        ];
+        $data[] = [
+            new external_single_structure([
+                'one' => new external_value(PARAM_INT),
+                'two' => new external_value(PARAM_INT),
+                'three' => new external_value(PARAM_INT),
+            ]),
+            ['one' => 1, 'two' => 2, 'three' => 3],
+            "<SINGLE>\n" .
+            "<KEY name=\"one\"><VALUE>1</VALUE>\n</KEY>\n" .
+            "<KEY name=\"two\"><VALUE>2</VALUE>\n</KEY>\n" .
+            "<KEY name=\"three\"><VALUE>3</VALUE>\n</KEY>\n" .
+            "</SINGLE>\n"
+        ];
+        $data[] = [ // Single structure with null value.
+            new external_single_structure([
+                'one' => new external_value(PARAM_INT),
+                'two' => new external_value(PARAM_INT),
+                'three' => new external_value(PARAM_INT),
+            ]),
+            ['one' => 1, 'two' => null, 'three' => 3],
+            "<SINGLE>\n" .
+            "<KEY name=\"one\"><VALUE>1</VALUE>\n</KEY>\n" .
+            "<KEY name=\"two\"><VALUE null=\"null\"/>\n</KEY>\n" .
+            "<KEY name=\"three\"><VALUE>3</VALUE>\n</KEY>\n" .
+            "</SINGLE>\n"
+        ];
+        $data[] = [ // Single structure missing keys.
+            new external_single_structure([
+                'one' => new external_value(PARAM_INT),
+                'two' => new external_value(PARAM_INT),
+                'three' => new external_value(PARAM_INT),
+            ]),
+            ['two' => null, 'three' => 3],
+            "<SINGLE>\n" .
+            "<KEY name=\"one\"><VALUE null=\"null\"/>\n</KEY>\n" .
+            "<KEY name=\"two\"><VALUE null=\"null\"/>\n</KEY>\n" .
+            "<KEY name=\"three\"><VALUE>3</VALUE>\n</KEY>\n" .
+            "</SINGLE>\n"
+        ];
+        $data[] = [ // Nested structure.
+            new external_single_structure([
+                'one' => new external_multiple_structure(
+                    new external_value(PARAM_INT)
+                ),
+                'two' => new external_multiple_structure(
+                    new external_single_structure([
+                        'firstname' => new external_value(PARAM_RAW),
+                        'lastname' => new external_value(PARAM_RAW),
+                    ])
+                ),
+                'three' => new external_single_structure([
+                    'firstname' => new external_value(PARAM_RAW),
+                    'lastname' => new external_value(PARAM_RAW),
+                ]),
+            ]),
+            [
+                'one' => [2, 3, 4],
+                'two' => [
+                    ['firstname' => 'Louis', 'lastname' => 'Armstrong'],
+                    ['firstname' => 'Neil', 'lastname' => 'Armstrong'],
+                ],
+                'three' => ['firstname' => 'Neil', 'lastname' => 'Armstrong'],
+            ],
+            "<SINGLE>\n" .
+            "<KEY name=\"one\"><MULTIPLE>\n".
+                "<VALUE>2</VALUE>\n" .
+                "<VALUE>3</VALUE>\n" .
+                "<VALUE>4</VALUE>\n" .
+            "</MULTIPLE>\n</KEY>\n" .
+            "<KEY name=\"two\"><MULTIPLE>\n".
+                "<SINGLE>\n" .
+                    "<KEY name=\"firstname\"><VALUE>Louis</VALUE>\n</KEY>\n" .
+                    "<KEY name=\"lastname\"><VALUE>Armstrong</VALUE>\n</KEY>\n" .
+                "</SINGLE>\n" .
+                "<SINGLE>\n" .
+                    "<KEY name=\"firstname\"><VALUE>Neil</VALUE>\n</KEY>\n" .
+                    "<KEY name=\"lastname\"><VALUE>Armstrong</VALUE>\n</KEY>\n" .
+                "</SINGLE>\n" .
+            "</MULTIPLE>\n</KEY>\n" .
+            "<KEY name=\"three\"><SINGLE>\n" .
+                "<KEY name=\"firstname\"><VALUE>Neil</VALUE>\n</KEY>\n" .
+                "<KEY name=\"lastname\"><VALUE>Armstrong</VALUE>\n</KEY>\n" .
+            "</SINGLE>\n</KEY>\n" .
+            "</SINGLE>\n"
+        ];
+        $data[] = [ // Nested structure with missing keys.
+            new external_single_structure([
+                'one' => new external_multiple_structure(
+                    new external_value(PARAM_INT)
+                ),
+                'two' => new external_multiple_structure(
+                    new external_single_structure([
+                        'firstname' => new external_value(PARAM_RAW),
+                        'lastname' => new external_value(PARAM_RAW),
+                    ])
+                ),
+                'three' => new external_single_structure([
+                    'firstname' => new external_value(PARAM_RAW),
+                    'lastname' => new external_value(PARAM_RAW),
+                ]),
+            ]),
+            [
+                'two' => [
+                    ['firstname' => 'Louis'],
+                    ['lastname' => 'Armstrong'],
+                ],
+                'three' => ['lastname' => 'Armstrong'],
+            ],
+            "<SINGLE>\n" .
+            "<KEY name=\"one\"><MULTIPLE>\n</MULTIPLE>\n</KEY>\n" .
+            "<KEY name=\"two\"><MULTIPLE>\n".
+                "<SINGLE>\n" .
+                    "<KEY name=\"firstname\"><VALUE>Louis</VALUE>\n</KEY>\n" .
+                    "<KEY name=\"lastname\"><VALUE null=\"null\"/>\n</KEY>\n" .
+                "</SINGLE>\n" .
+                "<SINGLE>\n" .
+                    "<KEY name=\"firstname\"><VALUE null=\"null\"/>\n</KEY>\n" .
+                    "<KEY name=\"lastname\"><VALUE>Armstrong</VALUE>\n</KEY>\n" .
+                "</SINGLE>\n" .
+            "</MULTIPLE>\n</KEY>\n" .
+            "<KEY name=\"three\"><SINGLE>\n" .
+                "<KEY name=\"firstname\"><VALUE null=\"null\"/>\n</KEY>\n" .
+                "<KEY name=\"lastname\"><VALUE>Armstrong</VALUE>\n</KEY>\n" .
+            "</SINGLE>\n</KEY>\n" .
+            "</SINGLE>\n"
+        ];
+        return $data;
+    }
+
+    /**
+     * @dataProvider xmlize_provider
+     * @param external_description $description The data structure.
+     * @param mixed $value The value to xmlise.
+     * @param mixed $expected The expected output.
+     */
+    public function test_xmlize($description, $value, $expected) {
+        $method = new ReflectionMethod('webservice_rest_server', 'xmlize_result');
+        $method->setAccessible(true);
+        $this->assertEquals($expected, $method->invoke(null, $value, $description));
+    }
+
+}