--- js/src/jsinterp.cpp.orig	2013-10-29 19:59:43.000000000 +0100
+++ js/src/jsinterp.cpp	2013-10-29 20:01:44.000000000 +0100
@@ -3664,7 +3664,9 @@
 BEGIN_CASE(JSOP_LEAVEFORLETIN)
 BEGIN_CASE(JSOP_LEAVEBLOCKEXPR)
 {
+#ifdef DEBUG
     DebugOnly<uint32_t> blockDepth = regs.fp()->blockChain().stackDepth();
+#endif
 
     regs.fp()->popBlock(cx);