diff --git a/dynax/system.py b/dynax/system.py
index a932f1d..c1f11d1 100644
--- a/dynax/system.py
+++ b/dynax/system.py
@@ -147,21 +147,22 @@ def __check_init__(self):
             if not hasattr(self, attr):
                 raise AttributeError(f"Attribute '{attr}' not initialized.")
 
-        # Check that vector_field and output returns Arrays or scalars and not PyTrees
-        x = self.initial_state
-        u = jax.ShapeDtypeStruct(dim2shape(self.n_inputs), jnp.float64)
-        try:
-            dx = eqx.filter_eval_shape(self.vector_field, x, u, t=1.0)
-            y = eqx.filter_eval_shape(self.output, x, u, t=1.0)
-        except Exception as e:
-            raise ValueError(
-                "Can not evaluate output shapes. Check your definitions!"
-            ) from e
-        for val, func in zip((dx, y), ("vector_field, output")):  # noqa: B905
-            if not isinstance(val, jax.ShapeDtypeStruct):
+        with jax.ensure_compile_time_eval():
+            # Check that vector_field and output returns Arrays or scalars - not PyTrees
+            x = jax.ShapeDtypeStruct(self.initial_state.shape, jnp.float64)
+            u = jax.ShapeDtypeStruct(dim2shape(self.n_inputs), jnp.float64)
+            try:
+                dx = eqx.filter_eval_shape(self.vector_field, x, u, t=1.0)
+                y = eqx.filter_eval_shape(self.output, x, u, t=1.0)
+            except Exception as e:
                 raise ValueError(
-                    f"{func} must return arrays or scalars, not {type(val)}"
-                )
+                    "Can not evaluate output shapes. Check your definitions!"
+                ) from e
+            for val, func in zip((dx, y), ("vector_field, output")):  # noqa: B905
+                if not isinstance(val, jax.ShapeDtypeStruct):
+                    raise ValueError(
+                        f"{func} must return arrays or scalars, not {type(val)}"
+                    )
 
     @abstractmethod
     def vector_field(
@@ -199,10 +200,12 @@ def output(
     @property
     def n_outputs(self) -> int | Literal["scalar"]:
         """The size of the output vector."""
-        x = self.initial_state
-        u = jax.ShapeDtypeStruct(dim2shape(self.n_inputs), jnp.float64)
-        y = eqx.filter_eval_shape(self.output, x, u, t=1.0)
-        return "scalar" if y.ndim == 0 else y.shape[0]
+        with jax.ensure_compile_time_eval():
+            x = jax.ShapeDtypeStruct(self.initial_state.shape, jnp.float64)
+            u = jax.ShapeDtypeStruct(dim2shape(self.n_inputs), jnp.float64)
+            y = eqx.filter_eval_shape(self.output, x, u, t=1.0)
+            n_out = "scalar" if y.ndim == 0 else y.shape[0]
+        return n_out
 
     def linearize(
         self,
@@ -309,24 +312,25 @@ class LinearSystem(AbstractControlAffine):
     """Feedthrough matrix."""
 
     def __post_init__(self):
-        self.initial_state = (
-            jnp.array(0) if self.A.ndim == 0 else jnp.zeros(self.A.shape[0])
-        )
-
-        if self.initial_state.ndim == 0:
-            if self.B.ndim == 0:
-                self.n_inputs = "scalar"
-            elif self.B.ndim == 1:
-                self.n_inputs = self.B.size
-            else:
-                raise ValueError("Dimension mismatch.")
-        else:
-            if self.B.ndim == 1:
-                self.n_inputs = "scalar"
-            elif self.B.ndim == 2:
-                self.n_inputs = self.B.shape[1]
+        # Without this context manager, `initial_state` will leak later
+        with jax.ensure_compile_time_eval():
+            self.initial_state = (
+                jnp.array(0) if self.A.ndim == 0 else jnp.zeros(self.A.shape[0])
+            )
+            if self.initial_state.ndim == 0:
+                if self.B.ndim == 0:
+                    self.n_inputs = "scalar"
+                elif self.B.ndim == 1:
+                    self.n_inputs = self.B.size
+                else:
+                    raise ValueError("Dimension mismatch.")
             else:
-                raise ValueError("Dimension mismatch.")
+                if self.B.ndim == 1:
+                    self.n_inputs = "scalar"
+                elif self.B.ndim == 2:
+                    self.n_inputs = self.B.shape[1]
+                else:
+                    raise ValueError("Dimension mismatch.")
 
     def f(self, x: Array) -> Array:
         return self.A.dot(x)