Undefined Symbol

Hey c0ders. Very Happy

I was trying to create my own dialog box, working from the example "DIALOG.ASM", just copied the code and left out what I thought I wouldn't need and kept in what I think would need. Allas I deleted a bit too much or included something that I should not include, I have a problem!

Here's the code:

Code:

format PE GUI 4.0
entry start

include 'win32a.inc'

section '.data' data readable writeable
  checkthis db 1Fh

section '.code' code readable executable

  start:
        invoke  GetModuleHandle,0
        invoke  DialogBoxParam,eax,37,HWND_DESKTOP,DialogProc, 0
        or      eax,eax
        jz      exit
  exit:
        invoke  ExitProcess,0

proc DialogProc hwnddlg,msg,wparam,lparam
        push    ebx esi edi
        cmp     [msg],WM_INITDIALOG
        je      wminitdialog
        cmp     [msg],WM_COMMAND
        je      wmcommand
        cmp     [msg],WM_CLOSE
        je      wmclose
        xor     eax,eax
        jmp     finish
  wminitdialog:
        jmp     processed
  wmcommand:
        cmp     [wparam],BN_CLICKED shl 16 + IDEXIT
        je      wmclose
        cmp     [wparam],BN_CLICKED shl 16 + IDOK
        jmp     processed
  wmclose:
        invoke  EndDialog,[hwnddlg], 0
  processed:
        mov     eax, 1
  finish:
        pop     edi esi ebx
        ret

endp

section '.idata' import data readable writeable

  library kernel,'KERNEL32.DLL',\
          user,'USER32.DLL'

  import  kernel,\
          GetModuleHandle,'GetModuleHandleA',\
          ExitProcess,'ExitProcess'

  import  user,\
          DialogBoxParam,'DialogBoxParamA',\
          GetDlgItemText,'GetDlgItemTextA',\
          MessageBox,'MessageBoxA',\
          EndDialog,'EndDialog'

section  '.rsrc' resource data readable

  directory RT_DIALOG,dialogs

  resource dialogs,\
           37, LANG_ENGLISH+SUBLANG_DEFAULT,mydialog

  dialog mydialog,'Clockowl - First Dialogbox',6,6,239,44,10CF0000h
    dialogitem 'EDIT','Yeey, it works!',IDTEXT,4,5,230,15,00000001h
    dialogitem 'BUTTON','&Ok',IDOK,152,25,82,13,50010000h
    dialogitem 'BUTTON','E&xit',IDEXIT,4,25,82,13,50010000h
  enddialog

As the title says, I get an undefined symbol error at the wmcommand: first line (which is line 24).
I don't get what the error means (not completely, I think it says I forgot to initialize something in the .data section (or anywhere else for that matter) but I can't figure out what)

Thanks in advance,
Daedalus