diff --git a/src/documents/serialisers.py b/src/documents/serialisers.py
index 3894181f4..d163e769d 100644
--- a/src/documents/serialisers.py
+++ b/src/documents/serialisers.py
@@ -645,26 +645,10 @@ class CustomFieldInstanceSerializer(serializers.ModelSerializer):
             custom_field.data_type,
         )
 
-        api_version = int(
-            self.context.get("request").version
-            if self.context.get("request")
-            else settings.REST_FRAMEWORK["DEFAULT_VERSION"],
-        )
-
         if custom_field.data_type == CustomField.FieldDataType.DOCUMENTLINK:
             # prior to update so we can look for any docs that are going to be removed
             self.reflect_doclinks(document, custom_field, validated_data["value"])
 
-        if (
-            custom_field.data_type == CustomField.FieldDataType.SELECT
-            and api_version < 7
-        ):
-            # Convert the index of the option in the field.extra_data["select_options"] list
-            # to the actual value
-            validated_data["value"] = custom_field.extra_data["select_options"][
-                validated_data["value"]
-            ]["id"]
-
         # Actually update or create the instance, providing the value
         # to fill in the correct attribute based on the type
         instance, _ = CustomFieldInstance.objects.update_or_create(
@@ -702,6 +686,12 @@ class CustomFieldInstanceSerializer(serializers.ModelSerializer):
         """
         data = super().validate(data)
         field: CustomField = data["field"]
+        api_version = int(
+            self.context.get("request").version
+            if self.context.get("request")
+            else settings.REST_FRAMEWORK["DEFAULT_VERSION"],
+        )
+
         if "value" in data and data["value"] is not None:
             if (
                 field.data_type == CustomField.FieldDataType.URL
@@ -729,6 +719,14 @@ class CustomFieldInstanceSerializer(serializers.ModelSerializer):
                 MaxLengthValidator(limit_value=128)(data["value"])
             elif field.data_type == CustomField.FieldDataType.SELECT:
                 select_options = field.extra_data["select_options"]
+
+                if api_version < 7:
+                    # Convert the index of the option in the field.extra_data["select_options"]
+                    # list to the options unique id
+                    data["value"] = field.extra_data["select_options"][data["value"]][
+                        "id"
+                    ]
+
                 try:
                     next(
                         option
diff --git a/src/documents/tests/test_api_custom_fields.py b/src/documents/tests/test_api_custom_fields.py
index 0479c103a..8cb42aa8d 100644
--- a/src/documents/tests/test_api_custom_fields.py
+++ b/src/documents/tests/test_api_custom_fields.py
@@ -356,23 +356,22 @@ class TestCustomFieldsAPI(DirectoriesMixin, APITestCase):
             checksum="123",
             mime_type="application/pdf",
         )
-        CustomFieldInstance.objects.create(
-            document=doc,
-            field=custom_field_select,
-            value_select="abc-123",
-        )
 
         resp = self.client.patch(
             f"/api/documents/{doc.id}/",
             headers={"Accept": "application/json; version=6"},
-            data={
-                "custom_fields": [
-                    {"field": custom_field_select.id, "value": 0},
-                ],
-            },
+            data=json.dumps(
+                {
+                    "custom_fields": [
+                        {"field": custom_field_select.id, "value": 1},
+                    ],
+                },
+            ),
+            content_type="application/json",
         )
         self.assertEqual(resp.status_code, status.HTTP_200_OK)
-        self.assertEqual(doc.custom_fields.first().value, "abc-123")
+        doc.refresh_from_db()
+        self.assertEqual(doc.custom_fields.first().value, "def-456")
 
         resp = self.client.get(
             f"/api/documents/{doc.id}/",
@@ -381,7 +380,7 @@ class TestCustomFieldsAPI(DirectoriesMixin, APITestCase):
         self.assertEqual(resp.status_code, status.HTTP_200_OK)
 
         data = resp.json()
-        self.assertEqual(data["custom_fields"][0]["value"], 0)
+        self.assertEqual(data["custom_fields"][0]["value"], 1)
 
     def test_create_custom_field_monetary_validation(self):
         """
@@ -980,6 +979,7 @@ class TestCustomFieldsAPI(DirectoriesMixin, APITestCase):
 
         resp = self.client.patch(
             f"/api/documents/{doc.id}/",
+            headers={"Accept": "application/json; version=7"},
             data={
                 "custom_fields": [
                     {"field": custom_field_select.id, "value": "not an option"},