diff --git a/sdcflows/interfaces/bspline.py b/sdcflows/interfaces/bspline.py
index 5674bdd486..0ba4ed98f7 100644
--- a/sdcflows/interfaces/bspline.py
+++ b/sdcflows/interfaces/bspline.py
@@ -198,7 +198,7 @@ def _run_interface(self, runtime):
 
         # Calculate collocation matrix from (possibly resized) image and knot grids
         colmat = sparse_hstack(
-            grid_bspline_weights(fmapnii, grid) for grid in bs_grids
+            [grid_bspline_weights(fmapnii, grid) for grid in bs_grids]
         ).tocsr()
 
         bs_grids_str = ["x".join(str(s) for s in grid.shape) for grid in bs_grids]