diff --git a/js/src/jsobj.cpp b/js/src/jsobj.cpp
index bb50c022e46..8d546c8ebc6 100644
--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@@ -548,6 +548,19 @@ js::SetIntegrityLevel(JSContext* cx, HandleObject obj, IntegrityLevel level)
 
         MOZ_ASSERT(nobj->lastProperty()->slotSpan() == last->slotSpan());
         JS_ALWAYS_TRUE(nobj->setLastProperty(cx, last));
+
+        // Ordinarily ArraySetLength handles this, but we're going behind its back
+        // right now, so we must do this manually.
+        //
+        // ArraySetLength also implements the capacity <= length invariant for
+        // arrays with non-writable length.  We don't need to do anything special
+        // for that, because capacity was zeroed out by preventExtensions.  (See
+        // the assertion about getDenseCapacity above.)
+        if (level == IntegrityLevel::Frozen && obj->is<ArrayObject>()) {
+            if (!obj->as<ArrayObject>().maybeCopyElementsForWrite(cx))
+                return false;
+            obj->as<ArrayObject>().getElementsHeader()->setNonwritableArrayLength();
+        }
     } else {
         RootedId id(cx);
         Rooted<PropertyDescriptor> desc(cx);
@@ -586,21 +599,6 @@ js::SetIntegrityLevel(JSContext* cx, HandleObject obj, IntegrityLevel level)
         }
     }
 
-    // Ordinarily ArraySetLength handles this, but we're going behind its back
-    // right now, so we must do this manually.  Neither the custom property
-    // tree mutations nor the DefineProperty call in the above code will do
-    // this for us.
-    //
-    // ArraySetLength also implements the capacity <= length invariant for
-    // arrays with non-writable length.  We don't need to do anything special
-    // for that, because capacity was zeroed out by preventExtensions.  (See
-    // the assertion before the if-else above.)
-    if (level == IntegrityLevel::Frozen && obj->is<ArrayObject>()) {
-        if (!obj->as<ArrayObject>().maybeCopyElementsForWrite(cx))
-            return false;
-        obj->as<ArrayObject>().getElementsHeader()->setNonwritableArrayLength();
-    }
-
     return true;
 }