diff --git a/bayesflow/trainers.py b/bayesflow/trainers.py
index d4779bef..16fb9f2b 100644
--- a/bayesflow/trainers.py
+++ b/bayesflow/trainers.py
@@ -21,9 +21,9 @@
 import logging
 import os
 from pickle import load as pickle_load
-import tensorflow as tf
 
 import numpy as np
+import tensorflow as tf
 from tqdm.autonotebook import tqdm
 
 from bayesflow.amortizers import (
@@ -737,7 +737,10 @@ def train_from_presimulation(
                     input_dict = self.configurator(epoch_data[index])
 
                     # Like the number of iterations, the batch size is inferred from presimulated dictionary or list
-                    batch_size = epoch_data[index][DEFAULT_KEYS["sim_data"]].shape[0]
+                    if isinstance(self.amortizer, AmortizedModelComparison):
+                        batch_size = input_dict[DEFAULT_KEYS["summary_conditions"]].shape[0]
+                    else:
+                        batch_size = epoch_data[index][DEFAULT_KEYS["sim_data"]].shape[0]
                     loss = self._train_step(batch_size, _backprop_step, input_dict, **kwargs)
 
                     # Store returned loss