diff --git a/src/CelerisLab/lbm/refinement_stepper.py b/src/CelerisLab/lbm/refinement_stepper.py
index 58ec5d3..cb649a6 100644
--- a/src/CelerisLab/lbm/refinement_stepper.py
+++ b/src/CelerisLab/lbm/refinement_stepper.py
@@ -134,6 +134,7 @@ class FixedPatchStepper:
             b = self.fine_boundary
             self._actuated = any(obj.surface_actuation is not None for obj in b.objects)
             if self._actuated:
+                fine_cfg.validate()  # derives nq etc.; the unique-writer key needs them
                 fluid_idx, dirs, _, body_id, rx, ry, _, _ = b.curved_host
                 self._fine_tables["surface_velocity"], _ = surface_link_velocity(
                     b.objects, fine_cfg, b.flags, fluid_idx, dirs, body_id, rx, ry)
diff --git a/tests/validation/multigrid/test_refinement_blocked_fine.py b/tests/validation/multigrid/test_refinement_blocked_fine.py
index 7f65274..06448ce 100644
--- a/tests/validation/multigrid/test_refinement_blocked_fine.py
+++ b/tests/validation/multigrid/test_refinement_blocked_fine.py
@@ -74,6 +74,7 @@ def fine_cfg(mode, patch):
     if mode == "slot_reservoir":
         cfg = replace(cfg, y_wall_bc="bounce_back", top_wall_bc="velocity",
                       wall_slot_width=0, wall_normal_velocity=0.0, velocity=0.0)
+    cfg.validate()  # as the stepper does before surface_link_velocity
     return cfg