diff --git a/src/CelerisLab/lbm/blocks.py b/src/CelerisLab/lbm/blocks.py
index a1aa588..f0fab2c 100644
--- a/src/CelerisLab/lbm/blocks.py
+++ b/src/CelerisLab/lbm/blocks.py
@@ -319,14 +319,18 @@ class BlockLayout2D:
         return blocked
 
     def to_dense(self, blocked):
-        """Unpack owner cores only; halo contents are ignored, holes are NaN."""
+        """Unpack owner cores only; halo contents are ignored, holes are NaN.
+
+        Full layouts accept any dtype; layouts with holes require floats.
+        """
         blocked = self._check_blocked(blocked)
         flat = blocked.reshape(self.nblocks, NQ, -1)
         owned = self.owned.ravel()
+        if not owned.all() and not np.issubdtype(blocked.dtype, np.floating):
+            raise ValueError("a layout with holes unpacks floating arrays only (holes are NaN)")
         ob, oc = np.divmod(self.owner.ravel()[owned].astype(np.intp), flat.shape[2])
         dense = np.empty((NQ, owned.size), dtype=blocked.dtype)
-        if not owned.all():  # full layouts keep any dtype; holes need a float NaN
-            dense[:, ~owned] = np.nan
+        dense[:, ~owned] = np.nan
         dense[:, owned] = flat[ob, :, oc].T
         return dense.reshape(NQ, *self.shape)
 
diff --git a/tests/unit/test_block_layout.py b/tests/unit/test_block_layout.py
index 9599c08..282931c 100644
--- a/tests/unit/test_block_layout.py
+++ b/tests/unit/test_block_layout.py
@@ -422,6 +422,18 @@ class SparseBlockLayoutTests(unittest.TestCase):
         self.assertEqual(layout.block_cell(0, 0, 0), address)
         self.assertEqual(address, layout.owner[0, 0])
 
+    def test_to_dense_dtype_contract(self):
+        full = BlockLayout2D.build(6, 8, 3, 4)
+        blocked = np.arange(np.prod(full.blocked_shape), dtype=np.int64).reshape(full.blocked_shape)
+        dense = full.to_dense(blocked)  # legacy: full layouts keep any dtype
+        self.assertEqual(dense.dtype, np.int64)
+        np.testing.assert_array_equal(dense, full.to_dense(blocked.astype(np.float64)))
+        sparse = BlockLayout2D.build(6, 8, 3, 4, origins=[(0, 0), (3, 4)])
+        for dtype in (np.int64, np.uint16, np.bool_):
+            with self.subTest(dtype=dtype), self.assertRaises(ValueError):
+                sparse.to_dense(np.zeros(sparse.blocked_shape, dtype=dtype))
+        self.assertEqual(sparse.to_dense(np.zeros(sparse.blocked_shape, np.float32)).dtype, np.float32)
+
     def test_invalid_origins_rejected(self):
         for origins in ([(0, 4), (0, 0)],          # not row-major
                         [(3, 0), (0, 4)],          # not row-major across rows