diff --git a/src/documents/serialisers.py b/src/documents/serialisers.py
index 9d8552e6f..c686a52f7 100644
--- a/src/documents/serialisers.py
+++ b/src/documents/serialisers.py
@@ -1668,9 +1668,8 @@ class PostDocumentSerializer(serializers.Serializer):
         max_value=Document.ARCHIVE_SERIAL_NUMBER_MAX,
     )
 
-    custom_fields = serializers.PrimaryKeyRelatedField(
-        many=True,
-        queryset=CustomField.objects.all(),
+    # Accept either a list of custom field ids or a dict mapping id -> value
+    custom_fields = serializers.JSONField(
         label="Custom fields",
         write_only=True,
         required=False,
@@ -1682,12 +1681,6 @@ class PostDocumentSerializer(serializers.Serializer):
         required=False,
     )
 
-    custom_fields_w_values = serializers.JSONField(
-        label="Custom fields with values",
-        write_only=True,
-        required=False,
-    )
-
     def validate_document(self, document):
         document_data = document.file.read()
         mime_type = magic.from_buffer(document_data, mime=True)
@@ -1733,33 +1726,61 @@ class PostDocumentSerializer(serializers.Serializer):
             return None
 
     def validate_custom_fields(self, custom_fields):
-        if custom_fields:
-            return [custom_field.id for custom_field in custom_fields]
-        else:
+        if not custom_fields:
             return None
 
-    def validate_custom_fields_w_values(self, custom_fields_w_values):
-        if custom_fields_w_values:
+        if isinstance(custom_fields, dict):
             custom_field_serializer = CustomFieldInstanceSerializer()
-            for field_id, value in custom_fields_w_values.items():
+            normalized = {}
+            for field_id, value in custom_fields.items():
                 try:
-                    field = CustomField.objects.get(id=field_id)
+                    field_id_int = int(field_id)
+                except (TypeError, ValueError):
+                    raise serializers.ValidationError(
+                        _("Custom field id must be an integer: %(id)s")
+                        % {"id": field_id},
+                    )
+
+                try:
+                    field = CustomField.objects.get(id=field_id_int)
                 except CustomField.DoesNotExist:
                     raise serializers.ValidationError(
                         _("Custom field with id %(id)s does not exist")
-                        % {
-                            "id": field_id,
-                        },
+                        % {"id": field_id_int},
                     )
-                # validate the value using the CustomFieldInstanceSerializer
+
                 custom_field_serializer.validate(
                     {
                         "field": field,
                         "value": value,
                     },
                 )
-            # Normalize keys to integers for later
-            return {int(k): v for k, v in custom_fields_w_values.items()}
+                normalized[field_id_int] = value
+
+            return normalized
+
+        if isinstance(custom_fields, list):
+            try:
+                ids = [int(i) for i in custom_fields]
+            except (TypeError, ValueError):
+                raise serializers.ValidationError(
+                    _(
+                        "Custom fields must be a list of integers or an object mapping ids to values.",
+                    ),
+                )
+            if CustomField.objects.filter(id__in=ids).count() != len(set(ids)):
+                raise serializers.ValidationError(
+                    _("Some custom fields don't exist or were specified twice."),
+                )
+            return ids
+
+        raise serializers.ValidationError(
+            _(
+                "Custom fields must be a list of integers or an object mapping ids to values.",
+            ),
+        )
+
+    # custom_fields_w_values handled via validate_custom_fields
 
     def validate_created(self, created):
         # support datetime format for created for backwards compatibility
diff --git a/src/documents/tests/test_api_documents.py b/src/documents/tests/test_api_documents.py
index bab03267d..48d52eca0 100644
--- a/src/documents/tests/test_api_documents.py
+++ b/src/documents/tests/test_api_documents.py
@@ -1554,7 +1554,7 @@ class TestDocumentApi(DirectoriesMixin, DocumentConsumeDelayMixin, APITestCase):
                 "/api/documents/post_document/",
                 {
                     "document": f,
-                    "custom_fields_w_values": json.dumps({"3456": "a string"}),
+                    "custom_fields": json.dumps({"3456": "a string"}),
                 },
             )
         self.assertEqual(response.status_code, status.HTTP_400_BAD_REQUEST)
@@ -1575,7 +1575,7 @@ class TestDocumentApi(DirectoriesMixin, DocumentConsumeDelayMixin, APITestCase):
                 "/api/documents/post_document/",
                 {
                     "document": f,
-                    "custom_fields_w_values": json.dumps(
+                    "custom_fields": json.dumps(
                         {
                             str(cf_string.id): "a string",
                             str(cf_int.id): 123,
diff --git a/src/documents/views.py b/src/documents/views.py
index 55e401a3a..7baf12c69 100644
--- a/src/documents/views.py
+++ b/src/documents/views.py
@@ -1497,10 +1497,7 @@ class PostDocumentView(GenericAPIView):
         title = serializer.validated_data.get("title")
         created = serializer.validated_data.get("created")
         archive_serial_number = serializer.validated_data.get("archive_serial_number")
-        custom_field_ids = serializer.validated_data.get("custom_fields")
-        custom_fields_w_values = serializer.validated_data.get(
-            "custom_fields_w_values",
-        )
+        cf = serializer.validated_data.get("custom_fields")
         from_webui = serializer.validated_data.get("from_webui")
 
         t = int(mktime(datetime.now().timetuple()))
@@ -1520,12 +1517,10 @@ class PostDocumentView(GenericAPIView):
             original_file=temp_file_path,
         )
         custom_fields = None
-        if custom_fields_w_values:
-            custom_fields = {
-                cf_id: value for cf_id, value in custom_fields_w_values.items()
-            }
-        elif custom_field_ids:
-            custom_fields = {cf_id: None for cf_id in custom_field_ids}
+        if isinstance(cf, dict) and cf:
+            custom_fields = {cf_id: value for cf_id, value in cf.items()}
+        elif isinstance(cf, list) and cf:
+            custom_fields = {cf_id: None for cf_id in cf}
         input_doc_overrides = DocumentMetadataOverrides(
             filename=doc_name,
             title=title,