diff --git a/tests/test_autobatching.py b/tests/test_autobatching.py
index 898bead0d..4bce41658 100644
--- a/tests/test_autobatching.py
+++ b/tests/test_autobatching.py
@@ -448,10 +448,20 @@ def test_in_flight_auto_batcher_restore_order(
     #     batcher.restore_original_order([si_sim_state])
 
 
+@pytest.mark.parametrize(
+    "num_steps_per_batch",
+    [
+        5,  # At 5 steps, not every state will converge before the next batch.
+        #       This tests the merging of partially converged states with new states
+        #       which has been a bug in the past. See https://github.com/Radical-AI/torch-sim/pull/219
+        10,  # At 10 steps, all states will converge before the next batch
+    ],
+)
 def test_in_flight_with_fire(
     si_sim_state: ts.SimState,
     fe_supercell_sim_state: ts.SimState,
     lj_model: LennardJonesModel,
+    num_steps_per_batch: int,
 ) -> None:
     fire_init, fire_update = unit_cell_fire(lj_model)
 
@@ -489,8 +499,7 @@ def convergence_fn(state: ts.SimState) -> bool:
         if state is None:
             break
 
-        # run 10 steps, arbitrary number
-        for _ in range(5):
+        for _ in range(num_steps_per_batch):
             state = fire_update(state)
         convergence_tensor = convergence_fn(state)
 
diff --git a/tests/test_state.py b/tests/test_state.py
index af0bda7b3..81109bf36 100644
--- a/tests/test_state.py
+++ b/tests/test_state.py
@@ -635,3 +635,16 @@ def test_deprecated_batch_properties_equal_to_new_system_properties(
     state.batch = new_system_idx
     assert torch.allclose(state.system_idx, new_system_idx)
     assert torch.allclose(state.batch, new_system_idx)
+
+
+def test_derived_classes_trigger_init_subclass() -> None:
+    """Test that derived classes cannot have attributes that are "tensors | None"."""
+
+    with pytest.raises(TypeError) as excinfo:
+
+        class DerivedState(SimState):
+            invalid_attr: torch.Tensor | None = None
+
+    assert "is not allowed to be of type 'torch.Tensor | None' because torch.cat" in str(
+        excinfo.value
+    )
diff --git a/torch_sim/optimizers.py b/torch_sim/optimizers.py
index 8626ff03f..9a41cd8c5 100644
--- a/torch_sim/optimizers.py
+++ b/torch_sim/optimizers.py
@@ -475,7 +475,7 @@ class FireState(SimState):
     # Required attributes not in SimState
     forces: torch.Tensor
     energy: torch.Tensor
-    velocities: torch.Tensor | None
+    velocities: torch.Tensor
 
     # FIRE algorithm parameters
     dt: torch.Tensor
@@ -972,7 +972,7 @@ class FrechetCellFIREState(SimState, DeformGradMixin):
 
     # Cell attributes
     cell_positions: torch.Tensor
-    cell_velocities: torch.Tensor | None
+    cell_velocities: torch.Tensor
     cell_forces: torch.Tensor
     cell_masses: torch.Tensor
 
diff --git a/torch_sim/runners.py b/torch_sim/runners.py
index 187cdd89b..8f2917da3 100644
--- a/torch_sim/runners.py
+++ b/torch_sim/runners.py
@@ -538,8 +538,8 @@ def static(
     @dataclass
     class StaticState(type(state)):
         energy: torch.Tensor
-        forces: torch.Tensor | None
-        stress: torch.Tensor | None
+        forces: torch.Tensor
+        stress: torch.Tensor
 
     all_props: list[dict[str, torch.Tensor]] = []
     og_filenames = trajectory_reporter.filenames
diff --git a/torch_sim/state.py b/torch_sim/state.py
index af4db6d19..d2ec83518 100644
--- a/torch_sim/state.py
+++ b/torch_sim/state.py
@@ -6,6 +6,7 @@
 
 import copy
 import importlib
+import typing
 import warnings
 from dataclasses import dataclass
 from typing import TYPE_CHECKING, Literal, Self, cast
@@ -400,6 +401,33 @@ def __getitem__(self, system_indices: int | list[int] | slice | torch.Tensor) ->
 
         return _slice_state(self, system_indices)
 
+    def __init_subclass__(cls, **kwargs) -> None:
+        """Enforce that all derived states cannot have tensor attributes that can also be
+        None. This is because torch.concatenate cannot concat between a tensor and a None.
+        See https://github.com/Radical-AI/torch-sim/pull/219 for more details.
+        """
+        # We need to use get_type_hints to correctly inspect the types
+        type_hints = typing.get_type_hints(cls)
+        for attr_name, attr_typehint in type_hints.items():
+            origin = typing.get_origin(attr_typehint)
+
+            is_union = origin is typing.Union
+            if not is_union and origin is not None:
+                # For Python 3.10+ `|` syntax, origin is types.UnionType
+                # We check by name to be robust against module reloading/patching issues
+                is_union = origin.__module__ == "types" and origin.__name__ == "UnionType"
+            if is_union:
+                args = typing.get_args(attr_typehint)
+                if torch.Tensor in args and type(None) in args:
+                    raise TypeError(
+                        f"Attribute '{attr_name}' in class '{cls.__name__}' is not "
+                        "allowed to be of type 'torch.Tensor | None' because torch.cat "
+                        "cannot concatenate between a tensor and a None. Please default "
+                        "the tensor with dummy values and track the 'None' case."
+                    )
+
+        super().__init_subclass__(**kwargs)
+
 
 class DeformGradMixin:
     """Mixin for states that support deformation gradients."""