diff --git a/processing/pre/geom_clean.py b/processing/pre/geom_clean.py
index 56e96d969..7520cf89d 100755
--- a/processing/pre/geom_clean.py
+++ b/processing/pre/geom_clean.py
@@ -76,7 +76,7 @@ for name in filenames:
 
 # --- do work ------------------------------------------------------------------------------------
 
-  microstructure = ndimage.filters.generic_filter(microstructure,mostFrequent,size=(options.stencil,options.stencil,options.stencil))
+  microstructure = ndimage.filters.generic_filter(microstructure,mostFrequent,size=(options.stencil,)*3)
 
 # --- write header ---------------------------------------------------------------------------------