--- a/js/public/StructuredClone.h
+++ b/js/public/StructuredClone.h
@@ -216,19 +216,19 @@ class MOZ_NON_MEMMOVABLE JS_PUBLIC_API(J
 {
     typedef js::SystemAllocPolicy AllocPolicy;
     typedef mozilla::BufferList<js::SystemAllocPolicy> BufferList;
 
     static const size_t kInitialSize = 0;
     static const size_t kInitialCapacity = 4096;
     static const size_t kStandardCapacity = 4096;
 
-    const JSStructuredCloneCallbacks* callbacks_;
-    void* closure_;
-    OwnTransferablePolicy ownTransferables_;
+    const JSStructuredCloneCallbacks* callbacks_ = nullptr;
+    void* closure_ = nullptr;
+    OwnTransferablePolicy ownTransferables_ = OwnTransferablePolicy::NoTransferables;
     js::SharedArrayRawBufferRefs refsHeld_;
 
     void setOptionalCallbacks(const JSStructuredCloneCallbacks* callbacks,
                               void* closure,
                               OwnTransferablePolicy policy) {
         callbacks_ = callbacks;
         closure_ = closure;
         ownTransferables_ = policy;