diff --git a/flexmeasures/data/models/generic_assets.py b/flexmeasures/data/models/generic_assets.py
index 3ba9330d4b..54ef2225e1 100644
--- a/flexmeasures/data/models/generic_assets.py
+++ b/flexmeasures/data/models/generic_assets.py
@@ -596,8 +596,9 @@ def search_beliefs(
                 )
                 df["sensor"] = {}  # ensure the same columns as a non-empty frame
             df = df.reset_index()
-            df["source"] = df["source"].apply(lambda x: x.to_dict())
-            df["sensor"] = df["sensor"].apply(lambda x: x.to_dict())
+            df["source"] = df["source"].ffill().apply(lambda x: x.to_dict())
+            df["sensor"] = df["sensor"].ffill().apply(lambda x: x.to_dict())
+
             return df.to_json(orient="records")
         return bdf_dict