diff --git a/js/accessibility/voicing/InteractiveHighlighting.ts b/js/accessibility/voicing/InteractiveHighlighting.ts
index a14d59f76..33374257d 100644
--- a/js/accessibility/voicing/InteractiveHighlighting.ts
+++ b/js/accessibility/voicing/InteractiveHighlighting.ts
@@ -486,7 +486,7 @@ const InteractiveHighlighting = memoize( <SuperType extends Constructor<Node>>(
    * NOTE: See Node's _mutatorKeys documentation for more information on how this operates, and potential special
    *       cases that may apply.
    */
-  InteractiveHighlightingClass.prototype._mutatorKeys = INTERACTIVE_HIGHLIGHTING_OPTIONS.concat( Type.prototype._mutatorKeys );
+  InteractiveHighlightingClass.prototype._mutatorKeys = INTERACTIVE_HIGHLIGHTING_OPTIONS.concat( InteractiveHighlightingClass.prototype._mutatorKeys );
 
   assert && assert( InteractiveHighlightingClass.prototype._mutatorKeys.length ===
                     _.uniq( InteractiveHighlightingClass.prototype._mutatorKeys ).length,
diff --git a/js/accessibility/voicing/Voicing.ts b/js/accessibility/voicing/Voicing.ts
index 1932842e8..d013ac368 100644
--- a/js/accessibility/voicing/Voicing.ts
+++ b/js/accessibility/voicing/Voicing.ts
@@ -648,7 +648,7 @@ const Voicing = <SuperType extends Constructor<Node>>( Type: SuperType ) => { //
    * NOTE: See Node's _mutatorKeys documentation for more information on how this operates, and potential special
    *       cases that may apply.
    */
-  VoicingClass.prototype._mutatorKeys = VOICING_OPTION_KEYS.concat( Type.prototype._mutatorKeys );
+  VoicingClass.prototype._mutatorKeys = VOICING_OPTION_KEYS.concat( VoicingClass.prototype._mutatorKeys );
 
   assert && assert( VoicingClass.prototype._mutatorKeys.length === _.uniq( VoicingClass.prototype._mutatorKeys ).length, 'duplicate mutator keys in Voicing' );