Debug Api

Hi. Forgive for my bad English and HELP ME PLEASE!!!

Why this code not working? Im testing with WinXP and Win7.

Debugger

Code:

include 'win32ax.inc'

        EXCEPTION_NONCONTINUABLE = 0x1
        EXCEPTION_MAXIMUM_PARAMETERS = 15
        EXCEPTION_DEBUG_EVENT = 1
        CREATE_THREAD_DEBUG_EVENT = 2
        CREATE_PROCESS_DEBUG_EVENT = 3
        EXIT_THREAD_DEBUG_EVENT = 4
        EXIT_PROCESS_DEBUG_EVENT = 5
        LOAD_DLL_DEBUG_EVENT = 6
        UNLOAD_DLL_DEBUG_EVENT = 7
        OUTPUT_DEBUG_STRING_EVENT = 8
        RIP_EVENT = 9

struct OUTPUT_DEBUG_STRING_INFO
  lpDebugStringData     dd ?;LPSTR
  fUnicode              dw ?;WORD 
  nDebugStringLength    dw ?;WORD 
ends

struct DEBUG_EVENT
  dwDebugEventCode dd ?
  dwProcessId      dd ?
  dwThreadId       dd ?
  u                rb 96-(3*4)
ends

        INFINITE equ 0FFFFFFFFh

        DBG_CONTINUE equ 0x00010002

.data
        hProcess        dd FALSE
        ProcessFinished dd TRUE
        BPAddr          dd FALSE
        RestoreBreak    dd FALSE

        NumbOfByteRead  dd ?


        startinfo       STARTUPINFO
        pi              PROCESS_INFORMATION
        DBEvent         DEBUG_EVENT

        buffer          rb MAX_PATH

.code
        start:
                stdcall OpenForDebug, <"app.exe">
        loop_debug:
                invoke  WaitForDebugEvent, DBEvent, INFINITE
                cmp     [DBEvent.dwDebugEventCode], EXIT_PROCESS_DEBUG_EVENT
                je      dbgapp_exit
                cmp     [DBEvent.dwDebugEventCode], CREATE_PROCESS_DEBUG_EVENT
                je      dbgapp_creat
                cmp     [DBEvent.dwDebugEventCode], OUTPUT_DEBUG_STRING_EVENT
                je      dbgapp_string
        continue_debug:
                invoke  ContinueDebugEvent, [DBEvent.dwProcessId], [DBEvent.dwThreadId], DBG_CONTINUE
                jmp     loop_debug
        dbgapp_string:
                mov     ebx, DBEvent.u
                virtual at 0
                        DebugString OUTPUT_DEBUG_STRING_INFO
                end virtual
                invoke  ReadProcessMemory, [hProcess], [ebx+DebugString.lpDebugStringData], buffer, [ebx+DebugString.nDebugStringLength], NumbOfByteRead
                invoke  MessageBox, 0, buffer , <"DEBUG APP OUT COMMAND"> , 0
                jmp     continue_debug
        dbgapp_exit:
              ;  invoke  MessageBox, 0, <"ExitProc">, 0, 0
                invoke  ContinueDebugEvent, [DBEvent.dwProcessId], [DBEvent.dwThreadId], DBG_CONTINUE
                jmp     close
        dbgapp_creat:
               ; invoke  MessageBox, 0, <"Event">, 0, 0
                jmp     continue_debug
        close:
                invoke  ExitProcess, 0

proc OpenForDebug, szFileName

        invoke  GetStartupInfo, startinfo
        invoke  CreateProcess, [szFileName], NULL, NULL, NULL, FALSE, PROCESS_QUERY_INFORMATION+DEBUG_PROCESS, NULL, NULL, startinfo, pi
        mov     eax, [pi.hProcess]
        mov     [hProcess],    eax
        ret
endp

.end start

and app for debug

Code:

include 'win32ax.inc'

.code
        start:
                invoke  MessageBox, 0, <"Hello World!">, <"Title">, MB_ICONINFORMATION
                invoke  OutputDebugString, <'invoke  MessageBox, 0, <"Hello World!">, <"Title">, MB_ICONINFORMATION'>
                invoke  ExitProcess, 0

.end start

hProcess is valid return in CreateProcess, In ReadProcessMemory raised exception (Access Violation) why? Please help me!!! Im not understand!!! Question