diff --git a/src/tracksdata/nodes/_mask.py b/src/tracksdata/nodes/_mask.py
index ea3cc56e..6743d12a 100644
--- a/src/tracksdata/nodes/_mask.py
+++ b/src/tracksdata/nodes/_mask.py
@@ -26,10 +26,10 @@ def _nd_sphere(
     """
 
     if ndim == 2:
-        return morph.disk(radius)
+        return morph.disk(radius).astype(bool)
 
     if ndim == 3:
-        return morph.ball(radius)
+        return morph.ball(radius).astype(bool)
 
     raise ValueError(f"Spherical is only implemented for 2D and 3D, got ndim={ndim}")
 
diff --git a/src/tracksdata/nodes/_test/test_mask.py b/src/tracksdata/nodes/_test/test_mask.py
index 6723dc44..d804f21d 100644
--- a/src/tracksdata/nodes/_test/test_mask.py
+++ b/src/tracksdata/nodes/_test/test_mask.py
@@ -376,6 +376,7 @@ def test_mask_from_coordinates_2d_basic() -> None:
     # Should be a disk of radius 2, shape (5,5), centered at (5,5)
     assert mask.mask.shape == (5, 5)
     assert mask.mask[2, 2]  # center pixel is True
+    assert mask.mask.dtype == bool
     np.testing.assert_array_equal(mask.bbox, [3, 3, 8, 8])