--- a/js/src/jsinterp.cpp
+++ b/js/src/jsinterp.cpp
@@ -761,20 +761,16 @@ InvokeSessionGuard::start(JSContext *cx,
     if (!stack.pushInvokeArgs(cx, argc, &args_))
         return false;
 
     /* Callees may clobber 'this' or 'callee'. */
     savedCallee_ = args_.callee() = calleev;
     savedThis_ = args_.thisv() = thisv;
 
     do {
-        /* In debug mode, script->getJIT(fp->isConstructing()) can change. */
-        if (cx->compartment->debugMode)
-            break;
-
         /* Hoist dynamic checks from scripted Invoke. */
         if (!calleev.isObject())
             break;
         JSObject &callee = calleev.toObject();
         if (callee.getClass() != &js_FunctionClass)
             break;
         JSFunction *fun = callee.getFunctionPrivate();
         if (fun->isNative())
@@ -793,17 +789,17 @@ InvokeSessionGuard::start(JSContext *cx,
 
 #ifdef JS_METHODJIT
         /* Hoist dynamic checks from RunScript. */
         mjit::CompileStatus status = mjit::CanMethodJIT(cx, script_, fp);
         if (status == mjit::Compile_Error)
             return false;
         if (status != mjit::Compile_Okay)
             break;
-        code_ = script_->getJIT(fp->isConstructing())->invokeEntry;
+        /* Cannot also cache the raw code pointer; it can change. */
 
         /* Hoist dynamic checks from CheckStackAndEnterMethodJIT. */
         JS_CHECK_RECURSION(cx, return JS_FALSE);
         stackLimit_ = stack.getStackLimit(cx);
         if (!stackLimit_)
             return false;
 
         stop_ = script_->code + script_->length - 1;

--- a/js/src/jsinterpinlines.h
+++ b/js/src/jsinterpinlines.h
@@ -535,17 +535,16 @@ PutActivationObjects(JSContext *cx, JSSt
 class InvokeSessionGuard
 {
     InvokeArgsGuard args_;
     InvokeFrameGuard frame_;
     Value savedCallee_, savedThis_;
     Value *formals_, *actuals_;
     unsigned nformals_;
     JSScript *script_;
-    void *code_;
     Value *stackLimit_;
     jsbytecode *stop_;
 
     bool optimized() const { return frame_.pushed(); }
 
   public:
     InvokeSessionGuard() : args_(), frame_() {}
     ~InvokeSessionGuard() {}
@@ -578,35 +577,33 @@ inline bool
 InvokeSessionGuard::invoke(JSContext *cx) const
 {
     /* N.B. Must be kept in sync with Invoke */
 
     /* Refer to canonical (callee, this) for optimized() sessions. */
     formals_[-2] = savedCallee_;
     formals_[-1] = savedThis_;
 
-    if (!optimized())
+    void *code;
+    if (!optimized() || !(code = script_->getJIT(false /* !constructing */)->invokeEntry))
         return Invoke(cx, args_, 0);
 
     /* Clear any garbage left from the last Invoke. */
     JSStackFrame *fp = frame_.fp();
     fp->clearMissingArgs();
     fp->resetInvokeCallFrame();
     SetValueRangeToUndefined(fp->slots(), script_->nfixed);
 
     JSBool ok;
     {
         AutoPreserveEnumerators preserve(cx);
         Probes::enterJSFun(cx, fp->fun(), script_);
 #ifdef JS_METHODJIT
-        if (code_ != script_->getJIT(fp->isConstructing())->invokeEntry)
-            *(volatile int *)0x101 = 0;
-
         AutoInterpPreparer prepareInterp(cx, script_);
-        ok = mjit::EnterMethodJIT(cx, fp, code_, stackLimit_);
+        ok = mjit::EnterMethodJIT(cx, fp, code, stackLimit_);
         cx->regs->pc = stop_;
 #else
         cx->regs->pc = script_->code;
         ok = Interpret(cx, cx->fp());
 #endif
         Probes::exitJSFun(cx, fp->fun(), script_);
     }