diff --git a/src/struphy/feec/psydac_derham.py b/src/struphy/feec/psydac_derham.py
index 7394b5b10..dfdb34e4c 100644
--- a/src/struphy/feec/psydac_derham.py
+++ b/src/struphy/feec/psydac_derham.py
@@ -2326,7 +2326,8 @@ def __init__(
         if xp.get_backend() == "cupy" and np.any((degree < 1) | (degree > 8)):
             raise ValueError("CUDA spline degrees must be between 1 and 8.")
         pn = xp.asarray(degree)
-        knots = tuple(xp.ascontiguousarray(t, dtype=float) for t in derham.V0fem.knots)
+        # the knots are host arrays; cupy.ascontiguousarray does not take NumPy arrays, xp.asarray copies them
+        knots = tuple(xp.asarray(np.ascontiguousarray(t, dtype=float)) for t in derham.V0fem.knots)
         starts = (self.starts,) if isinstance(self._vector_stencil, StencilVector) else self.starts
         kinds = derham.spline_attributes[self.space_key].spline_types_pyccel
         self._args_eval = tuple(
diff --git a/src/struphy/pic/sorting.py b/src/struphy/pic/sorting.py
index 2d331d6c1..74db8c182 100644
--- a/src/struphy/pic/sorting.py
+++ b/src/struphy/pic/sorting.py
@@ -1,4 +1,5 @@
 import logging
+import math
 from typing import TYPE_CHECKING
 
 import cunumpy as xp
@@ -226,7 +227,7 @@ def _set_boxes(self):
         n_particles = self._markers_shape[0]
         n_mkr = int(n_particles / n_box_in) + 1
         n_cols = round(
-            n_mkr * (1 + 1 / xp.sqrt(n_mkr) + self._box_bufsize),
+            n_mkr * (1 + 1 / math.sqrt(n_mkr) + self._box_bufsize),
         )
 
         # cartesian boxes (extra last row stores holes/outside particles)