diff --git a/addons/project/wizard/project_task_merge_wizard.py b/addons/project/wizard/project_task_merge_wizard.py
index 02ad9b7b1c5..1389a9d4899 100644
--- a/addons/project/wizard/project_task_merge_wizard.py
+++ b/addons/project/wizard/project_task_merge_wizard.py
@@ -63,7 +63,7 @@ class ProjectTaskMergeWizard(models.TransientModel):
         selected_tasks = self.env['project.task'].browse(self.env.context.get('active_ids', False))
         assigned_tasks = selected_tasks.filtered(lambda task: task.user_id)
         result.update({
-            'task_ids': [(4, task.id) for task in selected_tasks],
+            'task_ids': selected_tasks.ids,
             'user_id': assigned_tasks and assigned_tasks[0].user_id.id or False,
             'target_project_id': selected_tasks[0].project_id.id,
             'target_task_id': selected_tasks[0].id