Collecting handles with Windbg

I'm using Windbg sdk to write my own debugger. The debugger is capable of collecting all handles allocated by debugged apps to avoid handle leaks. Here is my code:

void zAdvancedDebugger::debugProc(){
    // Creating interfaces including m_dbgClient, m_dbgControl
    if(!createInterfaces()){
        printf("Failed to create debugging interfaces\r\n");
        return;
    }
    std::string strRealCmdLine="\"" + app + "\"  " + args;
    // Every thing's set up so start the app.
    if (( m_dbgClient->CreateProcess(0, (PSTR)strRealCmdLine.c_str(), DEBUG_PROCESS )) != S_OK)
        return ;
    // event loop
    while(true){
        if(m_dbgControl->WaitForEvent(DEBUG_WAIT_DEFAULT,INFINITE)!= S_OK)
            break;
    }
    HRESULT ret=m_dbgControl->Execute(
        DEBUG_OUTCTL_IGNORE,
        ".closehandle -a", // Close all handles allocated
        DEBUG_EXECUTE_NOT_LOGGED );
}

The problem is: I can't execute the command ".closehandle -a". Whenever I run the code, I always get "ret 0x80040205 An unexpected exception was raised". Would you please tell me what wrong is with this?