diff --git a/src/akkudoktoreos/core/pydantic.py b/src/akkudoktoreos/core/pydantic.py
index 6ab672e..18a6a07 100644
--- a/src/akkudoktoreos/core/pydantic.py
+++ b/src/akkudoktoreos/core/pydantic.py
@@ -1023,7 +1023,12 @@ class PydanticDateTimeDataFrame(PydanticBaseModel):
             return v
 
         # Validate consistent columns
-        columns = set(next(iter(v.values())).keys())
+        try:
+            columns = set(next(iter(v.values())).keys())
+        except AttributeError:
+            raise ValueError(
+                "Data values must be dicts (DataFrame format), not scalars. Use DateTimeData or DateTimeSeries format instead."
+            )
         if not all(set(row.keys()) == columns for row in v.values()):
             raise ValueError("All rows must have the same columns")