diff --git a/fet2020/tasks/forms.py b/fet2020/tasks/forms.py
index 40964b71..a0383427 100644
--- a/fet2020/tasks/forms.py
+++ b/fet2020/tasks/forms.py
@@ -1,6 +1,7 @@
 from django import forms
 from django.contrib.admin.widgets import FilteredSelectMultiple
 from django.contrib.auth.models import User
+from django.core.validators import ValidationError
 from django.forms.widgets import HiddenInput
 from django.utils import timezone
 from django.utils.translation import gettext_lazy as _
@@ -78,6 +79,24 @@ class TaskCreateForm(forms.ModelForm):
             "assigned_to"
         ].queryset.order_by("username")
 
+    def clean(self):
+        cleaned_data = super().clean()
+        assigned_to = cleaned_data["assigned_to"]
+
+        if assigned_to:
+            task_list = TaskList.objects.get(id=cleaned_data["task_list"].id)
+            for elem in task_list.users.all():
+                if assigned_to == elem:
+                    break
+            else:
+                raise ValidationError(
+                    _(
+                        f"User '{assigned_to}' gibt es nicht in der User-Liste der Task-Gruppe '{task_list}'."
+                    )
+                )
+
+        return cleaned_data
+
 
 class TaskUpdateForm(forms.ModelForm):
     class Meta:
@@ -137,7 +156,4 @@ class DocumentForm(forms.ModelForm):
             "date": _("Datum"),
         }
 
-        widgets = {
-            "date": DateInput(format=("%d-%m-%Y")),
-            "task": HiddenInput
-        }
+        widgets = {"date": DateInput(format=("%d-%m-%Y")), "task": HiddenInput}