diff --git a/grade/report/grader/index.php b/grade/report/grader/index.php
index 785e2efbe21..5146f6b6301 100644
--- a/grade/report/grader/index.php
+++ b/grade/report/grader/index.php
@@ -32,7 +32,7 @@ $courseid      = required_param('id', PARAM_INT);        // course id
 $page          = optional_param('page', 0, PARAM_INT);   // active page
 $edit          = optional_param('edit', -1, PARAM_BOOL); // sticky editting mode
 
-$sortitemid    = optional_param('sortitemid', 0, PARAM_ALPHANUM); // sort by which grade item
+$sortitemid    = optional_param('sortitemid', 0, PARAM_ALPHANUMEXT);
 $action        = optional_param('action', 0, PARAM_ALPHAEXT);
 $move          = optional_param('move', 0, PARAM_INT);
 $type          = optional_param('type', 0, PARAM_ALPHA);
diff --git a/grade/report/grader/lib.php b/grade/report/grader/lib.php
index c7ee6659a0c..9f0217a5625 100644
--- a/grade/report/grader/lib.php
+++ b/grade/report/grader/lib.php
@@ -442,20 +442,17 @@ class grade_report_grader extends grade_report {
             $sort = "g.finalgrade $this->sortorder, u.idnumber, u.lastname, u.firstname, u.email";
         } else {
             $sortjoin = '';
-            switch($this->sortitemid) {
-                case 'lastname':
-                    $sort = "u.lastname $this->sortorder, u.firstname $this->sortorder, u.idnumber, u.email";
-                    break;
-                case 'firstname':
-                    $sort = "u.firstname $this->sortorder, u.lastname $this->sortorder, u.idnumber, u.email";
-                    break;
-                case 'email':
-                    $sort = "u.email $this->sortorder, u.firstname, u.lastname, u.idnumber";
-                    break;
-                case 'idnumber':
-                default:
-                    $sort = "u.idnumber $this->sortorder, u.firstname, u.lastname, u.email";
-                    break;
+
+            // The default sort will be that provided by the site for users, unless a valid user field is requested,
+            // the value of which takes precedence.
+            [$sort] = users_order_by_sql('u', null, $this->context, $userfieldssql->mappings);
+            if (array_key_exists($this->sortitemid, $userfieldssql->mappings)) {
+
+                // Ensure user sort field doesn't duplicate one of the default sort fields.
+                $usersortfield = $userfieldssql->mappings[$this->sortitemid];
+                $defaultsortfields = array_diff(explode(', ', $sort), [$usersortfield]);
+
+                $sort = "{$usersortfield} {$this->sortorder}, " . implode(', ', $defaultsortfields);
             }
 
             $params = array_merge($gradebookrolesparams, $this->userwheresql_params, $this->groupwheresql_params, $enrolledparams, $relatedctxparams);
diff --git a/grade/report/grader/tests/behat/ajax_grader.feature b/grade/report/grader/tests/behat/ajax_grader.feature
index 7ca442a864c..de8115dab72 100644
--- a/grade/report/grader/tests/behat/ajax_grader.feature
+++ b/grade/report/grader/tests/behat/ajax_grader.feature
@@ -264,3 +264,9 @@ Feature: Using the AJAX grading feature of Grader report to update grades and fe
     When I navigate to "View > Grader report" in the course gradebook
     Then I should see "litle yellow frog" in the "student1" "table_row"
     And I should see "prince frog" in the "student2" "table_row"
+    # Sort by the custom profile field.
+    And I click on "Favourite frog" "link" in the "gradereport-grader-table" "table"
+    And "student1" "table_row" should appear before "student2" "table_row"
+    # Now sort by descending.
+    And I click on "Favourite frog" "link" in the "gradereport-grader-table" "table"
+    And "student2" "table_row" should appear before "student1" "table_row"