diff --git a/lib/bcoin/rbt.js b/lib/bcoin/rbt.js
index 364d09fb..709f68cd 100644
--- a/lib/bcoin/rbt.js
+++ b/lib/bcoin/rbt.js
@@ -363,6 +363,8 @@ RBT.prototype.rotr = function rotr(x) {
  */
 
 RBT.prototype.min = function min(z) {
+  if (z.isNull())
+    return z;
   while (!z.left.isNull())
     z = z.left;
   return z;
@@ -376,6 +378,8 @@ RBT.prototype.min = function min(z) {
  */
 
 RBT.prototype.max = function max(z) {
+  if (z.isNull())
+    return z;
   while (!z.right.isNull())
     z = z.right;
   return z;