diff --git a/frontend/src/app/util/object.cljc b/frontend/src/app/util/object.cljc
index 090effd710..9ff10e7b9f 100644
--- a/frontend/src/app/util/object.cljc
+++ b/frontend/src/app/util/object.cljc
@@ -466,10 +466,17 @@
 
 #?(:cljs
    (def Proxy
-     (app.util.object/class
-      :name "Proxy"
-      :extends js/Object
-      :constructor (constantly nil))))
+     (let [ctor (app.util.object/class
+                 :name "Proxy"
+                 :extends js/Object
+                 :constructor (constantly nil))]
+       ;; Remove the `constructor` data property from the prototype so that
+       ;; SES `harden` (used by the plugin sandbox) does not traverse from a
+       ;; proxy instance back to this constructor function and freeze it.
+       ;; If the constructor is frozen before Transit's `typeTag` helper sets
+       ;; its cache property, Transit throws "object is not extensible".
+       (js-delete (.-prototype ctor) "constructor")
+       ctor)))
 
 (defmacro reify
   "A domain specific variation of reify that creates anonymous objects