diff --git a/lib/legacy/legacy-data-mixin.html b/lib/legacy/legacy-data-mixin.html
index 3ef795cd1e..4e6bc06879 100644
--- a/lib/legacy/legacy-data-mixin.html
+++ b/lib/legacy/legacy-data-mixin.html
@@ -155,10 +155,12 @@
       Polymer.LegacyDataMixin(superClass)
   );
 
+  // Apply LegacyDataMixin to Templatizer instances as well, and defer
+  // runtime switch to the root's host (_methodHost)
   Polymer.Templatize.mixin = 
     Polymer.dedupingMixin(superClass => class extends Polymer.LegacyDataMixin(superClass) {
       get _legacyUndefinedCheck() {
-        return this._methodHost._legacyUndefinedCheck;
+        return this._methodHost && this._methodHost._legacyUndefinedCheck;
       }
     });