diff --git a/core/keyboard_nav/ast_node.js b/core/keyboard_nav/ast_node.js
index 332e9af8c..d86a22a4c 100644
--- a/core/keyboard_nav/ast_node.js
+++ b/core/keyboard_nav/ast_node.js
@@ -418,7 +418,7 @@ Blockly.ASTNode.prototype.findPrevForField_ = function() {
  */
 Blockly.ASTNode.prototype.navigateBetweenStacks_ = function(forward) {
   var curLocation = this.getLocation();
-  if (!(curLocation instanceof Blockly.Block)) {
+  if (curLocation.getSourceBlock) {
     curLocation = /** @type {!Blockly.IASTNodeLocationWithBlock} */ (
       curLocation).getSourceBlock();
   }