diff --git a/core/toolbox.js b/core/toolbox.js
index 3951c107d..f31f8840d 100644
--- a/core/toolbox.js
+++ b/core/toolbox.js
@@ -202,6 +202,10 @@ Blockly.Toolbox.prototype.populate_ = function(newTree) {
           var childOut = rootOut.createNode(childIn.getAttribute('name'));
           childOut.blocks = [];
           treeOut.add(childOut);
+          var customId = childIn.getAttribute('id');
+          if (customId) {
+            childOut.setId(customId);
+          }
           var custom = childIn.getAttribute('custom');
           if (custom) {
             // Variables and procedures are special dynamic categories.