diff --git a/CHANGELOG.md b/CHANGELOG.md
index 132974f5..44f58c6f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,9 +15,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - Fix flaky escalation test on solve/reject-solution data sets
 - CI: fix Psalm cache directory, drop the stray `glpi-project/tools` composer dependency, declare a unique composer autoloader suffix
+- Fix form submission error when 'Use technician group on ticket creation' is enabled
 - Simplify group reassignment to a single actor diff, relying on GLPI core's status preservation on simultaneous actor removal/addition
 
-
 ## [2.10.8] 2026-09-11
 
 ### Fixed
diff --git a/inc/ticket.class.php b/inc/ticket.class.php
index a52b3fe9..77059097 100644
--- a/inc/ticket.class.php
+++ b/inc/ticket.class.php
@@ -629,9 +629,11 @@ public static function assignUserGroup(Ticket $ticket)
             return false;
         }
 
+        // `_users_id_assign` may be either a single ID or an array of IDs
+        $users_id_assign = (int) current(array_filter((array) ($ticket->input['_users_id_assign'] ?? [])));
+
         if (
-            isset($ticket->input['_users_id_assign'])
-            && $ticket->input['_users_id_assign'] > 0
+            $users_id_assign > 0
             && (!isset($ticket->input['_groups_id_assign'])
                 || empty($ticket->input['_groups_id_assign']))
         ) {
@@ -640,7 +642,7 @@ public static function assignUserGroup(Ticket $ticket)
                 $ticket->input['_groups_id_assign']
                     = PluginEscaladeUser::getTechnicianGroup(
                         $ticket->input['entities_id'],
-                        current($ticket->input['_users_id_assign']),
+                        $users_id_assign,
                         true,
                     );
             } else {
@@ -648,7 +650,7 @@ public static function assignUserGroup(Ticket $ticket)
                 $ticket->input['_groups_id_assign']
                     = PluginEscaladeUser::getTechnicianGroup(
                         $ticket->input['entities_id'],
-                        current($ticket->input['_users_id_assign']),
+                        $users_id_assign,
                         false,
                     );
             }
diff --git a/tests/Units/TicketTest.php b/tests/Units/TicketTest.php
index 5531912e..019348a5 100644
--- a/tests/Units/TicketTest.php
+++ b/tests/Units/TicketTest.php
@@ -1724,4 +1724,60 @@ public function testKeepNewAssignUsersFlagIsClearedWhenFollowUpNeverHappens()
             'The keep-users flag must not survive past the request when the follow-up group add never happens',
         );
     }
+
+    /**
+     * When the technician is auto-assigned by the core from the ITIL category,
+     * `_users_id_assign` is a scalar ID (not an array): the technician group
+     * must still be assigned on ticket creation.
+     */
+    public function testAssignUserGroupOnCreationWithTechnicianFromCategory()
+    {
+        $this->initConfig([
+            'use_assign_user_group'          => 1,
+            'use_assign_user_group_creation' => 1,
+        ]);
+
+        $this->updateItem('Entity', 0, [
+            'auto_assign_mode' => Entity::AUTO_ASSIGN_CATEGORY_HARDWARE,
+        ]);
+
+        $tech = new User();
+        $tech->getFromDBbyName('tech');
+        $this->assertGreaterThan(0, $tech->getID());
+
+        $group = $this->createItem('Group', [
+            'name' => 'Technician group',
+            'entities_id' => 0,
+            'is_recursive' => 1,
+        ]);
+        $this->createItem('Group_User', [
+            'users_id' => $tech->getID(),
+            'groups_id' => $group->getID(),
+        ]);
+
+        $itil_cat = $this->createItem('ITILCategory', [
+            'name' => 'Cat with technician',
+            'users_id' => $tech->getID(),
+            'entities_id' => 0,
+        ]);
+
+        $ticket = $this->createItem('Ticket', [
+            'name' => 'Ticket without assigned technician',
+            'content' => 'Content',
+            'entities_id' => 0,
+            'itilcategories_id' => $itil_cat->getID(),
+        ]);
+        $ticket_id = $ticket->getID();
+
+        $this->assertEquals(
+            1,
+            countElementsInTable(Ticket_User::getTable(), ['tickets_id' => $ticket_id, 'type' => CommonITILActor::ASSIGN, 'users_id' => $tech->getID()]),
+            'Technician from the category should be assigned',
+        );
+        $this->assertEquals(
+            1,
+            countElementsInTable(Group_Ticket::getTable(), ['tickets_id' => $ticket_id, 'type' => CommonITILActor::ASSIGN, 'groups_id' => $group->getID()]),
+            'Technician group should be assigned',
+        );
+    }
 }