diff --git a/src/lib/polymer-bootstrap.html b/src/lib/polymer-bootstrap.html
index 3386d515bf..cdfc63a4a7 100644
--- a/src/lib/polymer-bootstrap.html
+++ b/src/lib/polymer-bootstrap.html
@@ -35,9 +35,13 @@
       // this may have resulted in a new prototype being created
       prototype = factory.prototype;
       var options = {
-        prototype: prototype,
-        extends: prototype.extends
+        prototype: prototype
       };
+      // NOTE: we're specifically supporting older Chrome versions here 
+      // (specifically Chrome 39) that throw when options.extends is undefined.
+      if (prototype.extends) {
+        options.extends = prototype.extends;
+      }
       Polymer.telemetry._registrate(prototype);
       document.registerElement(prototype.is, options);
       return factory;