diff --git a/swiftsimio/visualisation/smoothing_length/generate.py b/swiftsimio/visualisation/smoothing_length/generate.py
index 375709cf..517c6e90 100644
--- a/swiftsimio/visualisation/smoothing_length/generate.py
+++ b/swiftsimio/visualisation/smoothing_length/generate.py
@@ -18,6 +18,7 @@ def generate_smoothing_lengths(
     kernel_gamma: np.float32,
     neighbours: int = 32,
     speedup_fac: int = 2,
+    /,
     dimension: int = 3,
 ) -> cosmo_array:
     """