Fix #234

vessim/signal.py

@@ -68,6 +68,13 @@ def __init__(
         fill_method: Literal["ffill", "bfill"] = "ffill",
         column: Optional[str] = None,
     ):
+        if isinstance(actual, pd.DataFrame) and forecast is not None:
+            if isinstance(forecast, pd.DataFrame):
+                if not actual.columns.equals(forecast.columns):
+                    raise ValueError("Column names in actual and forecast do not match.")
+            else:
+                raise ValueError("Forecast has to be a DataFrame if actual is a DataFrame.")
+
         self.default_column = column
         self._fill_method = fill_method
         # Unpack index of actual dataframe
@@ -356,7 +363,8 @@ def _resample_to_frequency(
         else:
             # Actual value is used for interpolation
             times = np.insert(times, 0, start_time)
-            data = np.insert(data, 0, self.now(start_time, column))
+            # https://github.com/dos-group/vessim/issues/234
+            data = np.insert(data, 0, self.now(start_time, None if data.ndim == 1 else column))
             if resample_method == "ffill":
                 new_data = data[np.searchsorted(times, new_times, side="right") - 1]
             elif resample_method == "nearest":