Dll 释放内存的问题

我用编写了一个Dll程序,在ExitInstance()中释放内存
在程序调用该DLL后,在结束程序时为何出错
int CZyDbLibApp::ExitInstance()
{
// TODO: Add your specialized code here and/or call the base class
if (DbSystem) {
delete DbSystem;
} return CWinApp::ExitInstance();
}BOOL CZyDbLibApp::InitInstance()
{
// TODO: Add your specialized code here and/or call the base class
DbSystem = new CSystem();

return CWinApp::InitInstance();
}