diff --git a/lerobot/common/policies/diffusion/modeling_diffusion.py b/lerobot/common/policies/diffusion/modeling_diffusion.py
index 2a2a6554..1659b68e 100644
--- a/lerobot/common/policies/diffusion/modeling_diffusion.py
+++ b/lerobot/common/policies/diffusion/modeling_diffusion.py
@@ -19,6 +19,7 @@
 TODO(alexander-soare):
   - Remove reliance on Robomimic for SpatialSoftmax.
   - Remove reliance on diffusers for DDPMScheduler and LR scheduler.
+  - Make compatible with multiple image keys.
 """
 
 import math
@@ -85,7 +86,10 @@ class DiffusionPolicy(nn.Module, PyTorchModelHubMixin):
 
         image_keys = [k for k in config.input_shapes if k.startswith("observation.image")]
         # Note: This check is covered in the post-init of the config but have a sanity check just in case.
-        assert len(image_keys) == 1
+        if len(image_keys) != 1:
+            raise NotImplementedError(
+                f"{self.__class__.__name__} only handles one image for now. Got image keys {image_keys}."
+            )
         self.input_image_key = image_keys[0]
 
         self.reset()