diff --git a/src/documents/serialisers.py b/src/documents/serialisers.py
index add3610af..aaded56fe 100644
--- a/src/documents/serialisers.py
+++ b/src/documents/serialisers.py
@@ -1829,18 +1829,27 @@ class BulkEditSerializer(
                 f"Some custom fields in {name} don't exist or were specified twice.",
             )
 
-        if isinstance(custom_fields, dict):
-            custom_field_map = CustomField.objects.in_bulk(ids)
-            for raw_field_id, value in custom_fields.items():
-                field = custom_field_map.get(int(raw_field_id))
-                if (
-                    field is not None
-                    and field.data_type == CustomField.FieldDataType.DOCUMENTLINK
-                    and value is not None
-                ):
-                    if not isinstance(value, list):
-                        raise serializers.ValidationError("Value must be a list")
-                    validate_documentlink_targets(self.user, value)
+    def _validate_custom_field_values(self, custom_fields, name):
+        if not isinstance(custom_fields, dict):
+            return custom_fields
+
+        validated = {}
+        errors = {}
+        for raw_field_id, value in custom_fields.items():
+            field_id = int(raw_field_id)
+            validator = CustomFieldInstanceSerializer(
+                data={"field": field_id, "value": value},
+                context=self.context,
+            )
+            if validator.is_valid():
+                validated[field_id] = validator.validated_data["value"]
+            else:
+                errors[str(field_id)] = validator.errors
+
+        if errors:
+            raise serializers.ValidationError({name: errors})
+
+        return validated
 
     def validate_method(self, method):
         if method == "set_correspondent":
@@ -1944,6 +1953,10 @@ class BulkEditSerializer(
                 parameters["add_custom_fields"],
                 "add_custom_fields",
             )
+            parameters["add_custom_fields"] = self._validate_custom_field_values(
+                parameters["add_custom_fields"],
+                "add_custom_fields",
+            )
         else:
             raise serializers.ValidationError("add_custom_fields not specified")
 
diff --git a/src/documents/tests/test_api_bulk_edit.py b/src/documents/tests/test_api_bulk_edit.py
index e4272e78f..dd62f0c74 100644
--- a/src/documents/tests/test_api_bulk_edit.py
+++ b/src/documents/tests/test_api_bulk_edit.py
@@ -343,9 +343,33 @@ class TestBulkEditAPI(DirectoriesMixin, APITestCase):
         m.assert_called_once()
         args, kwargs = m.call_args
         self.assertListEqual(args[0], [self.doc1.id, self.doc3.id])
-        self.assertEqual(kwargs["add_custom_fields"], {str(self.cf1.id): "foo"})
+        self.assertEqual(kwargs["add_custom_fields"], {self.cf1.id: "foo"})
         self.assertEqual(kwargs["remove_custom_fields"], [self.cf2.id])
 
+    @mock.patch("documents.serialisers.bulk_edit.modify_custom_fields")
+    def test_api_modify_custom_fields_rejects_invalid_value(self, m) -> None:
+        self.setup_mock(m, "modify_custom_fields")
+
+        response = self.client.post(
+            "/api/documents/bulk_edit/",
+            json.dumps(
+                {
+                    "documents": [self.doc1.id],
+                    "method": "modify_custom_fields",
+                    "parameters": {
+                        "add_custom_fields": {self.cf1.id: "x" * 129},
+                        "remove_custom_fields": [],
+                    },
+                },
+            ),
+            content_type="application/json",
+        )
+
+        self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
+        self.assertIn("add_custom_fields", response.data)
+        self.assertIn(str(self.cf1.id), response.data["add_custom_fields"])
+        m.assert_not_called()
+
     @mock.patch("documents.serialisers.bulk_edit.modify_custom_fields")
     def test_api_modify_custom_fields_invalid_params(self, m) -> None:
         """