程序为什么执行后提示内存不能read啊（EasyX）

#include<graphics.h>
#include<conio.h>


void main()
{
      int x=320;
      setcolor(YELLOW);
         setfillstyle(GREEN);
         fillellipse(x,240,20,20);


         char c;
         while(c!=27)
         {
              c=getch();
              setcolor(BLACK);
              setfillstyle(BLACK);
              fillellipse(x,240,20,20);

              switch(c)
              {
              case 'a': x-=2;break;
              case 'd':x+=2;break;
              case 27:break;
              }

              setcolor(YELLOW);
              setfillstyle(GREEN);
              fillellipse(x,240,20,20);
              Sleep(10);
         }

         closegraph();
}