diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7b72ab2723..7be77e6419 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/)
 and this project adheres to [Semantic Versioning](http://semver.org/).
 
+## [UNRELEASED]
+
+- Fix error during generate associated material action
+
 ## [2.12.5] - 2026-01-08
 
 ### Fixed
diff --git a/ajax/linkactions.php b/ajax/linkactions.php
index ab395277a3..a35526b951 100644
--- a/ajax/linkactions.php
+++ b/ajax/linkactions.php
@@ -95,10 +95,22 @@
             break;
 
         case "show_group_by_entity":
+
+            if (isset($_POST['multiple']) && $_POST['multiple']) {
+                if (isset($_POST['value']) && !is_array($_POST['value'])) {
+                    $_POST['value'] = [$_POST['value']];
+                }
+
+                if (!isset($_POST['value'])) {
+                    $_POST['value'] = [];
+                }
+            }
+
             Group::dropdown(['name'      => "id[" . $_POST['id'] . "][groups_id]",
                 'entity'    => $_POST['entities'],
                 'value'  => $_POST['value'] ?? 0,
-                'condition' => ['is_assign' => 1],
+                'condition' => ['is_itemgroup' => 1],
+                'multiple' => $_POST['multiple'] ?? false,
             ]);
 
             if (isset($_POST['massaction'])) {
diff --git a/inc/link.class.php b/inc/link.class.php
index 92a8c491c6..a7b7953b1e 100644
--- a/inc/link.class.php
+++ b/inc/link.class.php
@@ -29,6 +29,7 @@
  */
 
 use Glpi\Application\View\TemplateRenderer;
+use Glpi\Features\AssignableItem;
 
 class PluginOrderLink extends CommonDBChild
 {
@@ -95,6 +96,7 @@ public function showItemGenerationForm($params)
         $i = 0;
         $item_rows = [];
         $found = false;
+        $itemtype = '';
         $order_web_dir = $CFG_GLPI['root_doc'] . '/plugins/order';
 
         foreach ($params["items"][self::class] as $key => $val) {
@@ -122,6 +124,7 @@ public function showItemGenerationForm($params)
                         : $order->fields["entities_id"],
                     'condition' => self::getCondition($itemtype),
                     'itemtype' => $itemtype,
+                    'assignableitem' => false,
                 ];
 
                 if ($templateID) {
@@ -132,15 +135,20 @@ public function showItemGenerationForm($params)
                     $row['otherserial'] = $item->fields["otherserial"] ?? "";
                     $row['states_id'] = $item->fields["states_id"] ?? "";
                     $row['locations_id'] = $item->fields["locations_id"] ?? "";
-                    $row['groups_id'] = $item->fields["groups_id"] ?? "";
+                    $row['groups_id'] = $item->fields["groups_id"] ?? [];
                     $row['immo_number'] = $item->fields["immo_number"] ?? "";
                     $row['template_name'] = $reference->getTemplateName($itemtype, $templateID);
+
+                    if (Toolbox::hasTrait($itemtype, AssignableItem::class)) {
+                        $row['assignableitem'] = true;
+                    }
+
                 } else {
                     $row['name'] = false;
                     $row['otherserial'] = false;
                     $row['states_id'] = false;
                     $row['locations_id'] = false;
-                    $row['groups_id'] = false;
+                    $row['groups_id'] = [];
                     $row['immo_number'] = false;
                     $row['template_name'] = "";
                 }
@@ -164,6 +172,7 @@ public function showItemGenerationForm($params)
             'active_entities' => $_SESSION['glpiactiveentities'] ?? [],
             'item_rows' => $item_rows,
             'order_web_dir' => $order_web_dir,
+            'assignableitem' => Toolbox::hasTrait($itemtype, AssignableItem::class),
         ]);
         return null;
     }
diff --git a/templates/generate_item.html.twig b/templates/generate_item.html.twig
index 882f6e572e..52c80bb57c 100644
--- a/templates/generate_item.html.twig
+++ b/templates/generate_item.html.twig
@@ -99,13 +99,20 @@
                 </td>
                 <td>
                     <span id='show_group_by_entity_id_mass'>
+
                         {{ fields.dropdownField(
                             'Group',
-                            "mass[groups_id]",
-                            0,
+                             "mass[groups_id]",
+                            assignableitem ? [] : 0,
                             "",
-                            fields_option|merge({'on_change': 'plugin_order_setAllGroups(this.value)'})
+                            fields_option|merge({
+                                'entity': -1,
+                                'condition': {'is_itemgroup': 1},
+                                'multiple': assignableitem ? true : false,
+                                'on_change': 'plugin_order_setAllGroups(this)'
+                            })
                         ) }}
+
                     </span>
                 </td>
                 <td>
@@ -233,7 +240,11 @@
                                 "id[" ~ item.i ~ "][groups_id]",
                                 item.groups_id,
                                 "",
-                                fields_option
+                                fields_option|merge({
+                                    'entity': -1,
+                                    'condition': {'is_itemgroup': 1},
+                                    'multiple': item.assignableitem ? true : false
+                                })
                             ) }}
                         </span>
                     </td>
@@ -279,6 +290,7 @@
             data: {
                 'entities': value,
                 'action': 'show_group_by_entity',
+                'multiple': {{ assignableitem ? 'true' : 'false' }},
                 'id': id
             },
             success: function(response) {
@@ -344,7 +356,12 @@
         }
     }
 
-    function plugin_order_setAllGroups(value) {
+    function plugin_order_setAllGroups(dom_select) {
+
+        const values = Array.from(dom_select.selectedOptions).map(
+            option => option.value
+        );
+
         var elements = document.querySelectorAll('[id^="show_group_by"]');
         for (var i = 0; i < elements.length; i++) {
             (function(index) {
@@ -355,7 +372,8 @@
                         'entities': 0,
                         'action': 'show_group_by_entity',
                         'id': index,
-                        'value': value
+                        'value': values,
+                        'multiple': {{ assignableitem ? 'true' : 'false' }}
                     },
                     success: function(response) {
                         $('#show_group_by_entity_id_' + index).html(response);