diff --git a/pkg/noun/jets/c/mas.c b/pkg/noun/jets/c/mas.c
index 4542647794..a024b51860 100644
--- a/pkg/noun/jets/c/mas.c
+++ b/pkg/noun/jets/c/mas.c
@@ -29,19 +29,14 @@ u3qc_mas(u3_atom a)
       return u3i_chub(a_d);
     }
     else {
-      c3_w len_w, top_w;
-      u3i_slab    sab_u;
+      u3i_slab sab_u;
+      u3i_slab_from(&sab_u, a, 0, --b_w);
 
-      u3i_slab_from(&sab_u, a, 0, b_w - 1);
-      len_w = sab_u.len_w - 1;
-      top_w = sab_u.buf_w[len_w];
+      sab_u.buf_w[(b_w >> 5)] &= ((c3_w)1 << (b_w & 31)) - 1;
+      b_w--;
+      sab_u.buf_w[(b_w >> 5)] |= ((c3_w)1 << (b_w & 31));
 
-      b_w   &= 31;
-      top_w &= ((c3_w)1 << (b_w - 1)) - 1;
-      top_w |= ((c3_w)1 << (b_w - 2));
-
-      sab_u.buf_w[len_w] = top_w;
-      return u3i_slab_moot(&sab_u);
+      return u3i_slab_mint(&sab_u);
     }
   }
 }