diff --git a/openavmkit/sales_scrutiny_study.py b/openavmkit/sales_scrutiny_study.py
index d981bc7..49da1d1 100644
--- a/openavmkit/sales_scrutiny_study.py
+++ b/openavmkit/sales_scrutiny_study.py
@@ -111,8 +111,8 @@ def __init__(self, df: pd.DataFrame, settings: dict, model_group: str):
         for key in stuff:
             df = stuff[key]
             df, cluster_fields = _mark_sales_scrutiny_clusters(df, settings)
-            df["ss_id"] = df["ss_id"].astype(str)
-            df["ss_id"] = df["model_group"] + "_" + key + "_" + df["ss_id"]
+            df["ss_id"] = df["ss_id"].astype(object).fillna("").astype(str)
+            df["ss_id"] = df["model_group"].astype(object).astype(str) + "_" + key + "_" + df["ss_id"]
             per_area = ""
             denominator = ""
             if key == "i":