diff --git a/klausur-service/backend/ocr_pipeline_api.py b/klausur-service/backend/ocr_pipeline_api.py
index e78084d..b898d0d 100644
--- a/klausur-service/backend/ocr_pipeline_api.py
+++ b/klausur-service/backend/ocr_pipeline_api.py
@@ -1600,7 +1600,7 @@ async def detect_rows(session_id: str):
     # Read zones from column_result to exclude box regions
     session = await get_session_db(session_id)
     column_result = (session or {}).get("column_result") or {}
-    zones = column_result.get("zones", [])
+    zones = column_result.get("zones") or []  # zones can be None for sub-sessions
 
     # Collect box y-ranges for filtering
     box_ranges = []  # [(y_start, y_end)]