diff --git a/lib/legacy/class.html b/lib/legacy/class.html
index 873b76c5e3..7506d2367b 100644
--- a/lib/legacy/class.html
+++ b/lib/legacy/class.html
@@ -54,7 +54,7 @@
       if (!Array.isArray(behaviors)) {
         behaviors = [behaviors];
       }
-      let superBehaviors = klass.behaviors;
+      let superBehaviors = klass.prototype.behaviors;
       // get flattened, deduped list of behaviors *not* already on super class
       behaviors = flattenBehaviors(behaviors, null, superBehaviors);
       // mixin new behaviors
@@ -62,6 +62,7 @@
       if (superBehaviors) {
         behaviors = superBehaviors.concat(behaviors);
       }
+      // Set behaviors on the prototype to be compatible with 1.x
       klass.prototype.behaviors = behaviors;
       return klass;
     }
@@ -320,8 +321,6 @@
         LegacyElementMixin(HTMLElement));
       // decorate klass with registration info
       klass.is = info.is;
-      // behaviors on prototype for BC...
-      klass.prototype.behaviors = klass.behaviors;
       // NOTE: while we could call `beforeRegister` here to maintain
       // some BC, the state of the element at this point is not as it was in 1.0
       // In 1.0, the method was called *after* mixing prototypes together