--- a/js/xpconnect/src/XPCCallContext.cpp
+++ b/js/xpconnect/src/XPCCallContext.cpp
@@ -10,17 +10,20 @@
 #include "jswrapper.h"
 #include "jsfriendapi.h"
 #include "nsContentUtils.h"
 
 using namespace mozilla;
 using namespace xpc;
 using namespace JS;
 
-#define IS_TEAROFF_CLASS(clazz) ((clazz) == &XPC_WN_Tearoff_JSClass)
+static inline bool IsTearoffClass(const js::Class* clazz)
+{
+    return clazz == &XPC_WN_Tearoff_JSClass;
+}
 
 XPCCallContext::XPCCallContext(JSContext* cx,
                                HandleObject obj    /* = nullptr               */,
                                HandleObject funobj /* = nullptr               */,
                                HandleId name       /* = JSID_VOID             */,
                                unsigned argc       /* = NO_ARGS               */,
                                Value* argv         /* = nullptr               */,
                                Value* rval         /* = nullptr               */)
@@ -59,17 +62,17 @@ XPCCallContext::XPCCallContext(JSContext
     if (!unwrapped) {
         JS_ReportErrorASCII(mJSContext, "Permission denied to call method on |this|");
         mState = INIT_FAILED;
         return;
     }
     const js::Class* clasp = js::GetObjectClass(unwrapped);
     if (IS_WN_CLASS(clasp)) {
         mWrapper = XPCWrappedNative::Get(unwrapped);
-    } else if (IS_TEAROFF_CLASS(clasp)) {
+    } else if (IsTearoffClass(clasp)) {
         mTearOff = (XPCWrappedNativeTearOff*)js::GetObjectPrivate(unwrapped);
         mWrapper = XPCWrappedNative::Get(
           &js::GetReservedSlot(unwrapped,
                                XPC_WN_TEAROFF_FLAT_OBJECT_SLOT).toObject());
     }
     if (mWrapper && !mTearOff) {
         mScriptable = mWrapper->GetScriptable();
     }