diff --git a/src/quartz_api/internal/service/uk_national/endpoint_types.py b/src/quartz_api/internal/service/uk_national/endpoint_types.py
index 6c9a77f..ebdd878 100644
--- a/src/quartz_api/internal/service/uk_national/endpoint_types.py
+++ b/src/quartz_api/internal/service/uk_national/endpoint_types.py
@@ -41,6 +41,9 @@ def convert_list_of_gsp_ids(gsp_ids:str | None) -> list[int] | None:
     if gsp_ids is None:
         return gsp_ids
 
+    if gsp_ids == "":
+        return None
+
     return [int(gsp_id) for gsp_id in gsp_ids.split(",")]
 
 class EnhancedBaseModel(BaseModel):
diff --git a/src/quartz_api/internal/service/uk_national/gsp_router.py b/src/quartz_api/internal/service/uk_national/gsp_router.py
index 418f30d..4fa595c 100644
--- a/src/quartz_api/internal/service/uk_national/gsp_router.py
+++ b/src/quartz_api/internal/service/uk_national/gsp_router.py
@@ -329,6 +329,7 @@ async def get_all_available_forecasts(
     """
     # Default (no gsp_ids): served from warm cache only. If we're here it's a cache miss —
     # trigger a warm in the background and ask the client to retry.
+    gsp_ids = convert_list_of_gsp_ids(gsp_ids)
 
     start_datetime_utc_set = start_datetime_utc != default_now_window_start()
     end_datetime_utc_set = end_datetime_utc != default_window_end()
@@ -365,7 +366,7 @@ async def get_all_available_forecasts(
         # gsp_ids path: custom query, fetch live.
         gsps_to_convert: dict[int, models.Location] = {
             k: v for k, v in gsp_id_map.items()
-            if k in convert_list_of_gsp_ids(gsp_ids)
+            if k in gsp_ids
         }
         tasks = [
                 db.get_predicted_generation(
@@ -567,10 +568,8 @@ async def get_truths_for_all_gsps(
     - **start_datetime_utc**: optional start datetime for the query.
     - **end_datetime_utc**: optional end datetime for the query.
     """
-    # Why not just make the endpoint type list[int] and let fast API do this?
-    gsp_ids: list[int] | None = convert_list_of_gsp_ids(gsp_ids)
     out: list[GSPYieldGroupByDatetime] = []
-
+    gsp_ids = convert_list_of_gsp_ids(gsp_ids)
     gsp_uuid_id_map: dict[UUID, int] = {v.uuid: k for k, v in gsp_id_map.items()}
 
     if gsp_ids is None: