diff --git a/js/layout/HeightSizable.ts b/js/layout/HeightSizable.ts
index 4e62508dd..77db4219a 100644
--- a/js/layout/HeightSizable.ts
+++ b/js/layout/HeightSizable.ts
@@ -353,8 +353,8 @@ const HeightSizable = memoize( <SuperType extends Constructor<Node>>( type: Supe
   } );
 
   // If we're extending into a Node type, include option keys
-  if ( type.prototype._mutatorKeys ) {
-    const existingKeys = type.prototype._mutatorKeys;
+  if ( HeightSizableTrait.prototype._mutatorKeys ) {
+    const existingKeys = HeightSizableTrait.prototype._mutatorKeys;
     const newKeys = HEIGHT_SIZABLE_OPTION_KEYS;
     const indexOfBoundsBasedOptions = existingKeys.indexOf( REQUIRES_BOUNDS_OPTION_KEYS[ 0 ] );
     HeightSizableTrait.prototype._mutatorKeys = [
diff --git a/js/layout/Sizable.ts b/js/layout/Sizable.ts
index 72cb7a8c9..c5f7613bc 100644
--- a/js/layout/Sizable.ts
+++ b/js/layout/Sizable.ts
@@ -342,8 +342,8 @@ const Sizable = memoize( <SuperType extends Constructor<Node>>( type: SuperType
   } );
 
   // If we're extending into a Node type, include option keys
-  if ( SuperExtendedType.prototype._mutatorKeys ) {
-    const existingKeys = SuperExtendedType.prototype._mutatorKeys;
+  if ( SizableTrait.prototype._mutatorKeys ) {
+    const existingKeys = SizableTrait.prototype._mutatorKeys;
     const newKeys = SIZABLE_SELF_OPTION_KEYS;
     const indexOfBoundsBasedOptions = existingKeys.indexOf( REQUIRES_BOUNDS_OPTION_KEYS[ 0 ] );
     SizableTrait.prototype._mutatorKeys = [
diff --git a/js/layout/WidthSizable.ts b/js/layout/WidthSizable.ts
index 2f4f6e951..9211ebfd8 100644
--- a/js/layout/WidthSizable.ts
+++ b/js/layout/WidthSizable.ts
@@ -355,8 +355,8 @@ const WidthSizable = memoize( <SuperType extends Constructor<Node>>( type: Super
   } );
 
   // If we're extending into a Node type, include option keys
-  if ( type.prototype._mutatorKeys ) {
-    const existingKeys = type.prototype._mutatorKeys;
+  if ( WidthSizableTrait.prototype._mutatorKeys ) {
+    const existingKeys = WidthSizableTrait.prototype._mutatorKeys;
     const newKeys = WIDTH_SIZABLE_OPTION_KEYS;
     const indexOfBoundsBasedOptions = existingKeys.indexOf( REQUIRES_BOUNDS_OPTION_KEYS[ 0 ] );
     WidthSizableTrait.prototype._mutatorKeys = [