diff --git a/packages/readers_extra/src/weathergen/readers_extra/registry.py b/packages/readers_extra/src/weathergen/readers_extra/registry.py
index 303e87ab9..b13ee4655 100644
--- a/packages/readers_extra/src/weathergen/readers_extra/registry.py
+++ b/packages/readers_extra/src/weathergen/readers_extra/registry.py
@@ -28,5 +28,9 @@ def get_extra_reader(stream_type: str) -> object | None:
             from weathergen.readers_extra.data_reader_anemoi_operan import DataReaderAnemoiOperan
 
             return DataReaderAnemoiOperan
+        case "fesom":
+            from weathergen.readers_extra.data_reader_fesom import DataReaderFesom
+
+            return DataReaderFesom
         case _:
             return None
diff --git a/src/weathergen/datasets/data_reader_obs.py b/src/weathergen/datasets/data_reader_obs.py
index 62b1dcfba..d5547ebfb 100644
--- a/src/weathergen/datasets/data_reader_obs.py
+++ b/src/weathergen/datasets/data_reader_obs.py
@@ -200,7 +200,7 @@ def _setup_sample_index(self) -> None:
                 self.indices_start = np.append(
                     self.indices_start,
                     np.ones(
-                        (diff_in_hours_end - self.hrly_index.shape[0] - 1) // step_hrs, dtype=int
+                        (diff_in_hours_end - (self.hrly_index.shape[0] - 1)) // step_hrs, dtype=int
                     )
                     * self.indices_start[-1],
                 )