Assert Failing and I don't know why c++

if(!((CPU2.peek()).isExecuting())) {
((PCB)CPU2.peek()).execute();
assert((CPU2.peek()).isExecuting());




   bool isExecuting() const
   {
      return (running == 1);
   }
   void execute()
   {
      running = 1;
   }

The assertion is failing, and I have no idea why... Please help, thanks Top is main function, bottom is PCB.h; 'CPU' class is a Heap