diff --git a/src/moscot/base/problems/_mixins.py b/src/moscot/base/problems/_mixins.py
index 059a112bb..aaa7dd459 100644
--- a/src/moscot/base/problems/_mixins.py
+++ b/src/moscot/base/problems/_mixins.py
@@ -264,6 +264,9 @@ def _cell_transition_online(
             "scale_by_marginals": False,
             "key_added": None,
         }
+        df_to, df_from = (df_target, df_source) if forward else (df_source, df_target)
+        df_to = df_to[res_annotation_key]
+
         if aggregation_mode == "annotation":
             func = partial(
                 move_op,
@@ -271,11 +274,10 @@ def _cell_transition_online(
                 split_mass=False,
                 **move_op_const_kwargs,
             )
-            df = (df_target if forward else df_source)[res_annotation_key]
             tm = self._annotation_aggregation_transition(  # type: ignore[attr-defined]
                 annotations_1=source_annotations_verified if forward else target_annotations_verified,
                 annotations_2=target_annotations_verified if forward else source_annotations_verified,
-                df=df,
+                df=df_to,
                 func=func,
             )
 
@@ -287,9 +289,8 @@ def _cell_transition_online(
                 **move_op_const_kwargs,
             )
             tm = self._cell_aggregation_transition(  # type: ignore[attr-defined]
-                df_from=df_source if forward else df_target,
-                df_to=df_target if forward else df_source,
-                annotation_key=res_annotation_key,
+                df_from=df_from,
+                df_to=df_to,
                 annotations=target_annotations_verified if forward else source_annotations_verified,
                 batch_size=batch_size,
                 func=func,
@@ -495,7 +496,6 @@ def _flatten(self: AnalysisMixinProtocol[K, B], data: dict[K, ArrayLike], *, key
 
     def _cell_aggregation_transition(
         self: AnalysisMixinProtocol[K, B],
-        annotation_key: str,
         df_from: pd.DataFrame,
         df_to: pd.DataFrame,
         annotations: list[Any],
@@ -504,7 +504,7 @@ def _cell_aggregation_transition(
     ) -> pd.DataFrame:
 
         # Factorize annotations in df_to
-        annotations_in_df_to = df_to[annotation_key].values
+        annotations_in_df_to = df_to.values
         codes_to, uniques_to = pd.factorize(annotations_in_df_to)
         # Map annotations in 'annotations' to codes
         annotations_to_code = {annotation: idx for idx, annotation in enumerate(uniques_to)}