diff --git a/promgen/rest_v2.py b/promgen/rest_v2.py
index c7005c3c..062c65ab 100644
--- a/promgen/rest_v2.py
+++ b/promgen/rest_v2.py
@@ -206,10 +206,9 @@ def assign_user(self, request, id):
         serializer.is_valid(raise_exception=True)
 
         user = User.objects.get(id=serializer.validated_data["id"])
-        if not user.is_active:
-            raise ValidationError({"detail": "Cannot assign permissions to an inactive user."})
         content_type = ContentType.objects.get_for_model(object)
         permission = content_type.model + "_" + serializer.validated_data["role"].lower()
+        validators.validate_assign_perm(user, object, permission)
         user_object_perm = assign_perm(permission, user, object)
         return Response(
             serializers.UserObjectPermissionSerializer(user_object_perm).data,
@@ -285,6 +284,7 @@ def remove_user(self, request, id, user_id):
             request.query_params.get("remove_sub_permissions", "true").lower() == "true"
         )
         user = User.objects.get(id=user_id)
+        validators.validate_remove_perm(user, self.get_object())
         self.remove_perm(user, remove_sub_permissions)
         return Response(status=HTTPStatus.NO_CONTENT)
 
diff --git a/promgen/validators.py b/promgen/validators.py
index 89758ce6..ebecfe1c 100644
--- a/promgen/validators.py
+++ b/promgen/validators.py
@@ -6,6 +6,7 @@
 from dateutil import parser
 from django.core.exceptions import ValidationError
 from django.core.validators import RegexValidator, URLValidator
+from django.utils.translation import gettext as _
 
 # See definition of duration field
 # https://prometheus.io/docs/prometheus/latest/configuration/configuration/#configuration-file
@@ -102,3 +103,19 @@ def validate_utf8(value):
         value.encode("utf-8").decode("utf-8")
     except (UnicodeEncodeError, UnicodeDecodeError):
         raise ValidationError("Invalid UTF-8 string.")
+
+
+def validate_assign_perm(user, object, permission):
+    if not user.is_active:
+        raise ValidationError(_("Cannot assign permissions to an inactive user."))
+    if user == object.owner and permission not in ["service_admin", "project_admin"]:
+        raise ValidationError(
+            _("Cannot assign permission for the owner. The owner must have the ADMIN role.")
+        )
+
+
+def validate_remove_perm(user, object):
+    if user == object.owner:
+        raise ValidationError(
+            _("Cannot remove permissions for the owner. Please transfer ownership first.")
+        )
diff --git a/promgen/views.py b/promgen/views.py
index d63e8d16..08cdd03f 100644
--- a/promgen/views.py
+++ b/promgen/views.py
@@ -17,6 +17,7 @@
 from django.contrib.auth.mixins import LoginRequiredMixin
 from django.contrib.auth.models import User
 from django.contrib.contenttypes.models import ContentType
+from django.core.exceptions import ValidationError
 from django.core.paginator import EmptyPage, Paginator
 from django.db import transaction
 from django.db.models import Count, Prefetch, Q
@@ -52,6 +53,7 @@
 from promgen.forms import GroupMemberForm, UserPermissionForm
 from promgen.mixins import PromgenGuardianPermissionMixin
 from promgen.shortcuts import resolve_domain
+from promgen.validators import validate_assign_perm, validate_remove_perm
 
 logger = logging.getLogger(__name__)
 
@@ -1956,15 +1958,10 @@ def post(self, request):
         if "user" == permission_type:
             user = User.objects.get_by_natural_key(request.POST["username"])
 
-            # Prevent changing permissions for the owner of the object
-            if user == obj.owner and request.POST["permission"] not in self.permission_required:
-                messages.warning(
-                    request,
-                    _(
-                        "Cannot assign permission for the owner. "
-                        "The owner must have the ADMIN role."
-                    ),
-                )
+            try:
+                validate_assign_perm(user, obj, permission)
+            except ValidationError as e:
+                messages.error(request, e.message)
                 return redirect(request.POST["next"])
 
             assign_perm(permission, user, obj)
@@ -2006,12 +2003,10 @@ def post(self, request):
         if "user" == permission_type:
             user = User.objects.get_by_natural_key(request.POST["username"])
 
-            # Prevent removing permissions for the owner of the object
-            if user == obj.owner:
-                messages.warning(
-                    request,
-                    _("Cannot remove permissions for the owner. Please transfer ownership first."),
-                )
+            try:
+                validate_remove_perm(user, obj)
+            except ValidationError as e:
+                messages.error(request, e.message)
                 return redirect(request.POST["next"])
 
             self.delete_perm(user)