[interfacekit] A little help...

How do I get rid of this error message:
src/kits/interface/Window.cpp: In method `bool BWindow::NeedsUpdate()
const':
src/kits/interface/Window.cpp:1126: passing `const BLooper' as `this'
argument of `bool BLooper::Lock()' discards qualifiers
src/kits/interface/Window.cpp:1128: passing `const BWindow' as `this'
argument of `void BLooper::Unlock()' discards qualifiers


bool BWindow::NeedsUpdate() const{
     PortLink::ReplyData  replyData;

     serverLink->SetOpCode( AS_NEEDS_UPDATE );

     Lock();
     serverLink->FlushWithReply( &replyData );
     Unlock();

     delete replyData.buffer;

     return replyData.code == SERVER_TRUE;
}

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