diff --git a/src/tasks/service.ts b/src/tasks/service.ts
index 743e9cb..eaa6ac0 100644
--- a/src/tasks/service.ts
+++ b/src/tasks/service.ts
@@ -1,5 +1,5 @@
 import type { Database } from 'bun:sqlite';
-import { db } from '../db';
+import { db, ensureUserExists } from '../db';
 
 type CreateTaskInput = {
   description: string;
@@ -23,11 +23,16 @@ export class TaskService {
         VALUES (?, ?, ?, ?)
       `);
 
+      const ensuredCreator = ensureUserExists(task.created_by, this.dbInstance);
+      if (!ensuredCreator) {
+        throw new Error('No se pudo asegurar created_by');
+      }
+
       const runResult = insertTask.run(
         task.description,
         task.due_date ?? null,
         task.group_id ?? null,
-        task.created_by
+        ensuredCreator
       );
       const taskId = Number((runResult as any).lastInsertRowid);
 
@@ -37,12 +42,18 @@ export class TaskService {
           VALUES (?, ?, ?)
         `);
 
-        // Evitar duplicados por (task_id, user_id)
+        // Evitar duplicados por (task_id, user_id) tras asegurar usuarios
         const seen = new Set<string>();
         for (const a of assignments) {
-          if (seen.has(a.user_id)) continue;
-          seen.add(a.user_id);
-          insertAssignment.run(taskId, a.user_id, a.assigned_by);
+          const ensuredUser = ensureUserExists(a.user_id, this.dbInstance);
+          if (!ensuredUser) continue;
+          if (seen.has(ensuredUser)) continue;
+          seen.add(ensuredUser);
+
+          const ensuredAssigner =
+            ensureUserExists(a.assigned_by || ensuredCreator, this.dbInstance) || ensuredCreator;
+
+          insertAssignment.run(taskId, ensuredUser, ensuredAssigner);
         }
       }