diff --git a/src/Security/Member.php b/src/Security/Member.php
index 1e3262943c9..1e294f1fd2d 100644
--- a/src/Security/Member.php
+++ b/src/Security/Member.php
@@ -1188,15 +1188,15 @@ public static function map_in_groups($groups = null)
      *
      * If no groups are passed, all groups with CMS permissions will be used.
      *
-     * @param array $groups Groups to consider or NULL to use all groups with
+     * @param SS_List|array|null $groups Groups to consider or NULL to use all groups with
      *                      CMS permissions.
      * @return Map Returns a map of all members in the groups given that
      *                have CMS permissions.
      */
-    public static function mapInCMSGroups($groups = null)
+    public static function mapInCMSGroups(SS_List|array|null $groups = null): Map
     {
         // non-countable $groups will issue a warning when using count() in PHP 7.2+
-        if (!$groups) {
+        if ($groups === null) {
             $groups = [];
         }
 
@@ -1205,7 +1205,7 @@ public static function mapInCMSGroups($groups = null)
             return ArrayList::create()->map();
         }
 
-        if (count($groups ?? []) == 0) {
+        if (count($groups) === 0) {
             $perms = ['ADMIN', 'CMS_ACCESS_AssetAdmin'];
 
             if (class_exists(CMSMain::class)) {
@@ -1246,7 +1246,7 @@ public static function mapInCMSGroups($groups = null)
             ]);
         }
 
-        return $members->sort('"Member"."Surname", "Member"."FirstName"')->map();
+        return $members->sort('"Surname", "FirstName"')->map();
     }
 
 
diff --git a/tests/php/Security/MemberTest.php b/tests/php/Security/MemberTest.php
index 18aa8517446..47a9eb08062 100644
--- a/tests/php/Security/MemberTest.php
+++ b/tests/php/Security/MemberTest.php
@@ -2,6 +2,8 @@
 
 namespace SilverStripe\Security\Tests;
 
+use InvalidArgumentException;
+use SilverStripe\Admin\LeftAndMain;
 use SilverStripe\Control\Cookie;
 use SilverStripe\Core\Config\Config;
 use SilverStripe\Core\Convert;
@@ -12,6 +14,7 @@
 use SilverStripe\ORM\DataObject;
 use SilverStripe\ORM\DB;
 use SilverStripe\ORM\FieldType\DBDatetime;
+use SilverStripe\ORM\Map;
 use SilverStripe\ORM\ValidationException;
 use SilverStripe\ORM\ValidationResult;
 use SilverStripe\Security\Group;
@@ -1618,4 +1621,111 @@ public function testChangePasswordToBlankIsValidated()
         $result = $member->changePassword('');
         $this->assertFalse($result->isValid());
     }
+
+    public function testMapInCMSGroupsNoLeftAndMain()
+    {
+        if (class_exists(LeftAndMain::class)) {
+            $this->markTestSkipped('LeftAndMain must not exist for this test.');
+        }
+        $result = Member::mapInCMSGroups();
+        $this->assertInstanceOf(Map::class, $result);
+
+        $this->assertEmpty($result, 'Without LeftAndMain, no groups are CMS groups.');
+    }
+
+    /**
+     * @dataProvider mapInCMSGroupsProvider
+     */
+    public function testMapInCMSGroups(array $groupFixtures, array $groupCodes, array $expectedValue)
+    {
+        if (!empty($groupFixtures) && !empty($groupCodes)) {
+            throw new InvalidArgumentException('Data provider is misconfigured for this test.');
+        }
+
+        if (!class_exists(LeftAndMain::class)) {
+            $this->markTestSkipped('LeftAndMain must exist for this test.');
+        }
+
+        $groups = [];
+
+        // Convert fixture names to IDs
+        if (!empty($groupFixtures)) {
+            foreach ($groupFixtures as $index => $groupFixtureName) {
+                $groups[$index] = $this->objFromFixture(Group::class, $groupFixtureName)->ID;
+            }
+        }
+
+        // Convert codes to DataList
+        if (!empty($groupCodes)) {
+            $groups = Group::get()->filter(['Code' => $groupCodes]);
+        }
+
+        $result = Member::mapInCMSGroups($groups);
+        $this->assertInstanceOf(Map::class, $result);
+
+        $this->assertSame($expectedValue, $result->values());
+    }
+
+    public function mapInCMSGroupsProvider()
+    {
+        // Note: "ADMIN User" is not from the fixtures, that user is created by $this->logInWithPermission('ADMIN')
+        return [
+            'defaults' => [
+                'groupFixtures' => [],
+                'groupCodes' => [],
+                'expectedValue' => [
+                    'Admin ',
+                    'OtherAdmin ',
+                    'ADMIN User',
+                ],
+            ],
+            'single group in a list' => [
+                'groupFixtures' => [],
+                'groupCodes' => [
+                    'staffgroup'
+                ],
+                'expectedValue' => [
+                    'Staff User',
+                ],
+            ],
+            'single group in IDs array' => [
+                'groups' => [
+                    'staffgroup',
+                ],
+                'groupCodes' => [],
+                'expectedValue' => [
+                    'Staff User',
+                ],
+            ],
+            'multiple groups in a list' => [
+                'groupFixtures' => [],
+                'groupCodes' => [
+                    'staffgroup',
+                    'securityadminsgroup',
+                ],
+                'expectedValue' => [
+                    'Staff User',
+                    'Test User',
+                ],
+            ],
+            'multiple groups in IDs array' => [
+                'groupFixtures' => [
+                    'staffgroup',
+                    'securityadminsgroup',
+                ],
+                'groupCodes' => [],
+                'expectedValue' => [
+                    'Staff User',
+                    'Test User',
+                ],
+            ],
+            'group with no members' => [
+                'groupFixtures' => [],
+                'groupCodes' => [
+                    'memberless',
+                ],
+                'expectedValue' => [],
+            ],
+        ];
+    }
 }
diff --git a/tests/php/Security/MemberTest.yml b/tests/php/Security/MemberTest.yml
index 0f9ed3c9147..f67254ee038 100644
--- a/tests/php/Security/MemberTest.yml
+++ b/tests/php/Security/MemberTest.yml
@@ -58,6 +58,8 @@
     Email: noexpiry@silverstripe.com
     Password: 1nitialPassword
   staffmember:
+    FirstName: Staff
+    Surname: User
     Email: staffmember@test.com
     Groups: '=>SilverStripe\Security\Group.staffgroup'
   managementmember: