diff --git a/lib/db/upgrade.php b/lib/db/upgrade.php
index b130390cf1e..41755e62263 100644
--- a/lib/db/upgrade.php
+++ b/lib/db/upgrade.php
@@ -2864,6 +2864,28 @@ function xmldb_main_upgrade($oldversion) {
 
     // Automatically generated Moodle v3.3.0 release upgrade line.
     // Put any upgrade step following this.
+    if ($oldversion < 2017060800.01) {
+        // Data fix any assign group override event priorities which may have been accidentally nulled due to a bug on the group
+        // overrides edit form.
+
+        // First, find all assign group override events having null priority (and join their corresponding assign_overrides entry).
+        $sql = "SELECT e.id AS id, o.sortorder AS priority
+                  FROM {assign_overrides} o
+                  JOIN {event} e ON (e.modulename = 'assign' AND o.assignid = e.instance AND e.groupid = o.groupid)
+                 WHERE o.groupid IS NOT NULL AND e.priority IS NULL
+              ORDER BY o.id";
+        $affectedrs = $DB->get_recordset_sql($sql);
+
+        // Now update the event's priority based on the assign_overrides sortorder we found. This uses similar logic to
+        // assign_refresh_events(), except we've restricted the set of assignments and overrides we're dealing with here.
+        foreach ($affectedrs as $record) {
+            $DB->set_field('event', 'priority', $record->priority, ['id' => $record->id]);
+        }
+        $affectedrs->close();
+
+        // Main savepoint reached.
+        upgrade_main_savepoint(true, 2017060800.01);
+    }
 
     return true;
 }
diff --git a/mod/assign/override_form.php b/mod/assign/override_form.php
index b3cf8d4bb83..0c68b771006 100644
--- a/mod/assign/override_form.php
+++ b/mod/assign/override_form.php
@@ -55,6 +55,9 @@ class assign_override_form extends moodleform {
     /** @var int userid, if provided. */
     protected $userid;
 
+    /** @var int sortorder, if provided. */
+    protected $sortorder;
+
     /**
      * Constructor.
      * @param moodle_url $submiturl the form action URL.
@@ -72,6 +75,7 @@ class assign_override_form extends moodleform {
         $this->groupmode = $groupmode;
         $this->groupid = empty($override->groupid) ? 0 : $override->groupid;
         $this->userid = empty($override->userid) ? 0 : $override->userid;
+        $this->sortorder = empty($override->sortorder) ? null : $override->sortorder;
 
         parent::__construct($submiturl, null, 'post');
 
@@ -97,6 +101,10 @@ class assign_override_form extends moodleform {
                 $mform->addElement('select', 'groupid',
                         get_string('overridegroup', 'assign'), $groupchoices);
                 $mform->freeze('groupid');
+                // Add a sortorder element.
+                $mform->addElement('hidden', 'sortorder', $this->sortorder);
+                $mform->setType('sortorder', PARAM_INT);
+                $mform->freeze('sortorder');
             } else {
                 // Prepare the list of groups.
                 $groups = groups_get_all_groups($cm->course);
diff --git a/version.php b/version.php
index e90db3493dd..597192ca5a9 100644
--- a/version.php
+++ b/version.php
@@ -29,7 +29,7 @@
 
 defined('MOODLE_INTERNAL') || die();
 
-$version  = 2017060800.00;              // YYYYMMDD      = weekly release date of this DEV branch.
+$version  = 2017060800.01;              // YYYYMMDD      = weekly release date of this DEV branch.
                                         //         RR    = release increments - 00 in DEV branches.
                                         //           .XX = incremental changes.