diff --git a/arrays.scad b/arrays.scad
index 0e3ab97..5c0e7e7 100644
--- a/arrays.scad
+++ b/arrays.scad
@@ -1249,7 +1249,8 @@ function group_sort(list, idx) =
 //   small = list_smallest(list, k)
 // Description:
 //   Returns a set of the k smallest items in list in arbitrary order.  The items must be
-//   mutually comparable with native OpenSCAD comparison operations.
+//   mutually comparable with native OpenSCAD comparison operations.  You will get "undefined operation"
+//   errors if you provide invalid input. 
 // Arguments:
 //   list = list to process
 //   k = number of items to return
diff --git a/vectors.scad b/vectors.scad
index 4abdfe1..d9cf79c 100644
--- a/vectors.scad
+++ b/vectors.scad
@@ -494,7 +494,7 @@ function _bt_tree(points, ind, leafsize=25) =
         pmc    = mean(projc), 
         pivot  = min_index([for(p=projc) abs(p-pmc)]),
         radius = max([for(i=ind) norm(points[ind[pivot]]-points[i]) ]),
-        median = ninther(projc),
+        median = mean(projc),
         Lind   = [for(i=idx(ind)) if(projc[i]<=median && i!=pivot) ind[i] ],
         Rind   = [for(i=idx(ind)) if(projc[i] >median && i!=pivot) ind[i] ]
       )