diff --git a/src/CelerisLab/lbm/block_stepper.py b/src/CelerisLab/lbm/block_stepper.py
index c4c0cfe..76f2195 100644
--- a/src/CelerisLab/lbm/block_stepper.py
+++ b/src/CelerisLab/lbm/block_stepper.py
@@ -42,6 +42,10 @@ def _static_circles(layout, objects):
                 or any(getattr(obj.state, name) != 0 for name in ("vx", "vy", "omega"))
                 or any(vars(obj.control).values())):
             raise ValueError("Block bodies must be stationary passive circles")
+        cx, cy = obj.geometry.center
+        radius = obj.geometry._radius
+        if not np.isfinite([cx, cy, radius]).all() or radius <= 0:
+            raise ValueError("Block circle center/radius must be finite with radius > 0")
         mask = obj.geometry.build_flag_mask(nx, ny)
         solid = (mask & SOLID) != 0
         if not solid.any() or np.any(flags[solid] & SOLID):
@@ -93,7 +97,8 @@ class PeriodicBlockStepper:
         tpb = checked.threads_per_block
         if layout.nblocks > MAX_GRID_DIM or ly > MAX_GRID_DIM or not 0 < pairs < 2**31:
             raise ValueError("block count, block rows or halo pairs exceed launch limits")
-        self.boundary = _static_circles(layout, tuple(objects)) if objects else None
+        objects = tuple(objects)  # an empty iterable takes the body-free path
+        self.boundary = _static_circles(layout, objects) if objects else None
         links = 0 if self.boundary is None else len(self.boundary["cell"])
         if links >= 2**31:
             raise ValueError("cut links exceed launch limits")
diff --git a/tests/validation/multigrid/test_block_stepper.py b/tests/validation/multigrid/test_block_stepper.py
index 24e5634..ee84a49 100644
--- a/tests/validation/multigrid/test_block_stepper.py
+++ b/tests/validation/multigrid/test_block_stepper.py
@@ -134,6 +134,15 @@ class CpuContracts(unittest.TestCase):
         for objects in ([moving], [circle((1.5, 11.5), 2.0)], [circle(), circle()]):
             with self.assertRaises(ValueError):
                 PeriodicBlockStepper(config(), layout, context=self.forbidden_context(), objects=objects)
+        for center, radius in (((15.5, 11.5), -.1), ((15.5, 11.5), -.5), ((15.5, 11.5), np.nan),
+                               ((np.inf, 11.5), 4.3), ((15.5, np.nan), 4.3)):
+            with self.subTest(center=center, radius=radius), self.assertRaises(ValueError):
+                PeriodicBlockStepper(config(), layout, context=self.forbidden_context(),
+                                     objects=[circle(center, radius)])
+        with patch.object(implementation, "_static_circles") as prepare:
+            with self.assertRaisesRegex(AssertionError, "CUDA reached"):  # body-free path
+                PeriodicBlockStepper(config(), layout, context=self.forbidden_context(), objects=iter(()))
+            prepare.assert_not_called()
         with patch.object(CircleGeometry, "build_cut_links",
                           lambda geometry, *a, **k: build_links(geometry, *a, **k)[1:]):
             with self.assertRaisesRegex(ValueError, "cover every fluid pull"):