Failed to create a main window using Windows API and x86 assembly - winapi

Question

The program compiles fine, but it fails to create the main window. Specifically, CreateWindowEx fails and prints "Failed to create window".
Would anyone happen to know what I'm doing wrong? I'm following Kip Irvine's book on assembly almost exactly, but it seems I'm missing something.
EDIT: I updated the code based on the recommendations. Now the program fails to register the window class, and the specific error is that a "Parameter is incorrect". I looked over the parameters of my WNDCLASSEX struct, and couldn't fine anything wrong.
EDIT2: I removed the "Ex" from WNDCLASS and RegisterClass and the window shows up and works fine now. So I guess it was some kind of weird redefinition or inconsistency of structs and functions in the masm32rt library?
INCLUDE \masm32\include\masm32rt.inc
.data
windowName BYTE "ASM Windows App",0
className BYTE "ASMWin",0
MainWinClass WNDCLASSEX <NULL,CS_HREDRAW + CS_VREDRAW,WinProc,NULL,NULL,NULL,NULL,NULL,COLOR_WINDOW+1,NULL,className,NULL>
windowHandle DWORD ?
hInstance DWORD ?
.code
WinMain PROC
; Get a handle to the current process.
INVOKE GetModuleHandle, NULL
mov hInstance, eax
mov MainWinClass.hInstance, eax
; Check if the handle was received.
.IF eax == 0
pushad
print "Failed to get handle on current process"
popad
call ErrorHandler
jmp ExitProgram
.ENDIF
; Load the program's icon and cursor.
INVOKE LoadIcon, NULL, IDI_APPLICATION
mov MainWinClass.hIcon, eax
INVOKE LoadCursor, NULL, IDC_ARROW
mov MainWinClass.hCursor, eax
; Create the window class.
INVOKE RegisterClassEx, ADDR MainWinClass
; Check if the class was registered.
.IF eax == 0
pushad
print "Failed to register class."
popad
call ErrorHandler
jmp ExitProgram
.ENDIF
; Create the window.
INVOKE CreateWindowEx, 0, ADDR className, ADDR windowName, WS_VISIBLE,
CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
NULL, NULL, hInstance, NULL
; Check if window was created successfully.
.IF eax == 0
pushad
print "Failed to create window"
popad
call ErrorHandler
jmp ExitProgram
.ENDIF
; Save the window handle and use it to show the window.
mov windowHandle, eax
INVOKE ShowWindow, windowHandle, SW_SHOW
INVOKE UpdateWindow, windowHandle
; Message Loop
;MessageLoop:
; INVOKE GetMessage, ADDR msg, NULL, NULL, NULL
; INVOKE DispatchMessage, ADDR msg
; jmp MessageLoop
ExitProgram:
;CALL ReadChar
INVOKE ExitProcess, 0
WinMain ENDP
; Window Procedure
WinProc PROC,
hWnd:DWORD, localMsg:DWORD, wParam:DWORD, lParam:DWORD
mov eax, localMsg
.IF eax == WM_CREATE
;call WriteString
jmp WinProcExit
.ELSEIF eax == WM_CLOSE
;call WriteString
jmp WinProcExit
.ELSE
;call WriteString
INVOKE DefWindowProc, hWnd, localMsg, wParam, lParam
jmp WinProcExit
.ENDIF
WinProcExit:
ret
WinProc ENDP
ErrorHandler PROC
.data
pErrorMsg DWORD ?
messageID DWORD ?
.code
INVOKE GetLastError
mov messageID, eax
; Get the corresponding message string.
INVOKE FormatMessage, FORMAT_MESSAGE_ALLOCATE_BUFFER + \
FORMAT_MESSAGE_FROM_SYSTEM,NULL,messageID,NULL,
ADDR pErrorMsg,NULL,NULL
; Display the error message.
INVOKE MessageBox, NULL, pErrorMsg, NULL,
MB_ICONERROR+MB_OK
; Free the error message string.
INVOKE LocalFree, pErrorMsg
ret
ErrorHandler ENDP
END WinMain

Answer 1

WNDCLASSEX requires WNDCLASSEX.cbSize to be set. The mistake I made was that I assumed it could be NULL.
So I added this piece of code before registering the class:
; Initializing other parameters of the window class.
mov eax, SIZEOF MainWinClass
mov MainWinClass.cbSize, eax
In addition, Kip Irvine's functions cause errors when used along side some functions of the user interface section of the Windows API. I'm not exactly sure why that happens but it could be that some register values are changed around.