Fix

src/mrinufft/density/utils.py

@@ -56,8 +56,12 @@ def normalize_weights(weights):
 def normalize_density(kspace_loc, shape, density, backend, **kwargs):
     """Normalize the density to ensure that the reconstruction is stable."""
     from mrinufft import get_operator
-
+    xp = np
+    if backend == "cufinufft":
+        import cupy as cp
+        xp = cp
     test_op = get_operator(backend)(samples=kspace_loc, shape=shape, **kwargs)
-    test_im = np.ones(shape, dtype=test_op.cpx_dtype)
+    test_im = xp.ones(shape, dtype=test_op.cpx_dtype)
     test_im_recon = test_op.adj_op(density * test_op.op(test_im))
-    density /= np.mean(np.abs(test_im_recon))
+    density /= xp.mean(xp.abs(test_im_recon))
+    return density