diff --git a/core/toolbox.js b/core/toolbox.js
index 964a097a5..1fb5b5758 100644
--- a/core/toolbox.js
+++ b/core/toolbox.js
@@ -197,6 +197,10 @@ Blockly.Toolbox.prototype.populate_ = function(newTree) {
             childOut.setExpanded(true);
           }
           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.