diff --git a/tasks/models.py b/tasks/models.py
index 65d841d..7c1ea6d 100644
--- a/tasks/models.py
+++ b/tasks/models.py
@@ -54,6 +54,8 @@ def filter_by_symbol(cls, priority_filter):
         return cls.objects.filter(priority=priority_filter)
 
     def change_assignee(self, new_assignee):
+        if new_assignee not in User.objects.all():
+            raise Exception
         prev_assignee = self.assignee
         if new_assignee is None:
             raise TypeError("Valid user must be provided")
diff --git a/tasks/tests/test_change_assignee.py b/tasks/tests/test_change_assignee.py
index 8c0c2b5..916f436 100644
--- a/tasks/tests/test_change_assignee.py
+++ b/tasks/tests/test_change_assignee.py
@@ -1,6 +1,7 @@
 import pytest
 from tasks.models import Task
-from users.models import Team, User
+from users.models import Team, User, Role
+from django.contrib.auth.models import User as DjangoUser
 
 
 @pytest.mark.django_db
@@ -34,3 +35,20 @@ def test_change_assignee_other_team(self, test_db):
         with pytest.raises(Exception):
             task.change_assignee(employee_2)
         assert task.assignee == employee_1
+
+    @pytest.fixture
+    def out_db_user(self, test_db):
+        team = test_db[0][0]
+        django_user = DjangoUser.objects.create(username="testtest",
+                                                password="ssdalhSFDAJ23!",
+                                                email="example@redhat.com",
+                                                first_name="test",
+                                                last_name="test")
+        user = User(user=django_user, role=Role.EMPLOYEE, team=team)
+        return user
+
+    def test_change_assignee_not_db_user(self, test_db, out_db_user):
+        tasks = test_db[3]
+        task = tasks[0]
+        with pytest.raises(Exception):
+            task.change_assignee(out_db_user)