diff --git a/mozilla/xpcom/ds/nsArena.cpp b/mozilla/xpcom/ds/nsArena.cpp
index e1b3c6a0fca..b5018d38469 100644
--- a/mozilla/xpcom/ds/nsArena.cpp
+++ b/mozilla/xpcom/ds/nsArena.cpp
@@ -19,6 +19,12 @@
 #include "nsArena.h"
 #include "nsCRT.h"
 
+ArenaImpl::ArenaImpl(void)
+{
+  NS_INIT_REFCNT();
+  nsCRT::memset(&mPool, 0, sizeof(PLArenaPool));
+}
+
 NS_IMETHODIMP
 ArenaImpl::Init(PRUint32 aBlockSize)
 {
diff --git a/mozilla/xpcom/ds/nsArena.h b/mozilla/xpcom/ds/nsArena.h
index 7a7443915bd..b69f4e4a4d6 100644
--- a/mozilla/xpcom/ds/nsArena.h
+++ b/mozilla/xpcom/ds/nsArena.h
@@ -27,7 +27,7 @@
 // Simple arena implementation layered on plarena
 class ArenaImpl : public nsIArena {
 public:
-  ArenaImpl(void) { NS_INIT_REFCNT(); }
+  ArenaImpl(void);
   virtual ~ArenaImpl();
 
   NS_DECL_ISUPPORTS