diff --git a/src/gfn/containers/trajectories.py b/src/gfn/containers/trajectories.py
index 251b5921..ff8e7852 100644
--- a/src/gfn/containers/trajectories.py
+++ b/src/gfn/containers/trajectories.py
@@ -16,6 +16,11 @@
 from gfn.containers.transitions import Transitions
 
 
+def is_tensor(t) -> bool:
+    """Checks whether t is a torch.Tensor instance."""
+    return isinstance(t, Tensor)
+
+
 # TODO: remove env from this class?
 class Trajectories(Container):
     """Container for complete trajectories (starting in $s_0$ and ending in $s_f$).
diff --git a/src/gfn/env.py b/src/gfn/env.py
index 588b2553..9b045ca3 100644
--- a/src/gfn/env.py
+++ b/src/gfn/env.py
@@ -218,9 +218,7 @@ def _step(
         not_done_states = new_states[~new_sink_states_idx]
         not_done_actions = actions[~new_sink_states_idx]
 
-        new_not_done_states_tensor = self.maskless_step(
-            not_done_states, not_done_actions
-        )
+        new_not_done_states_tensor = self.step(not_done_states, not_done_actions)
 
         new_states.tensor[~new_sink_states_idx] = new_not_done_states_tensor