diff --git a/src/gen-vowel-constraints.py b/src/gen-vowel-constraints.py
index 02a338f..c4e75aa 100755
--- a/src/gen-vowel-constraints.py
+++ b/src/gen-vowel-constraints.py
@@ -176,9 +176,9 @@
 print ()
 
 print ('void')
-print ('_hb_preprocess_text_vowel_constraints (const hb_ot_shape_plan_t *plan,')
+print ('_hb_preprocess_text_vowel_constraints (const hb_ot_shape_plan_t *plan HB_UNUSED,')
 print ('\t\t\t\t       hb_buffer_t              *buffer,')
-print ('\t\t\t\t       hb_font_t                *font)')
+print ('\t\t\t\t       hb_font_t                *font HB_UNUSED)')
 print ('{')
 print ('  /* UGLY UGLY UGLY business of adding dotted-circle in the middle of')
 print ('   * vowel-sequences that look like another vowel.  Data for each script')