Window not created but GetLast returns success

[TotallyGamerJet]

I'm probably doing something stupid. But I'm trying to replicate this
Windows tutorial.
But when I run this code RegisterClassEx fails returning 0. Not sure if this helps but it never calls my WndProc function. (I place a Printf on the first line)

 package main

 import (
     "fmt"
     "github.com/lxn/win"
     "golang.org/x/sys/windows"
 )

 func main() {
     className := "SomeFunkyNameHere"

     wc := win.WNDCLASSEX{}

     hInst := win.GetModuleHandle(nil)
     if hInst == 0 {
	     panic("GetModuleHandle")
     }

     wc.LpfnWndProc = windows.NewCallback(WndProc)
     wc.HInstance = hInst
     wc.LpszClassName = windows.StringToUTF16Ptr(className)

     if atom := win.RegisterClassEx(&wc); atom == 0 {
	     panic("RegisterClassEx")
     }

     hwnd := win.CreateWindowEx(
	     0,
	     wc.LpszClassName,
	     windows.StringToUTF16Ptr("Learn to program"),
	     win.WS_OVERLAPPEDWINDOW,
	     win.CW_USEDEFAULT, win.CW_USEDEFAULT, win.CW_USEDEFAULT, win.CW_USEDEFAULT,
	     0,
       	     0,
	     hInst,
	    nil,
     )
     if hwnd == 0 {
	     panic(win.GetLastError())
     }

     win.ShowWindow(hwnd, win.SW_SHOW)
     msg := win.MSG{}
     for win.GetMessage(&msg, 0, 0, 0) != 0 {
	     win.TranslateMessage(&msg)
	     win.DispatchMessage(&msg)
     }
}

 func WndProc(hwnd win.HWND, msg uint32, wParam, lParam uintptr) uintptr {
     switch msg {
     case win.WM_DESTROY:
	    win.PostQuitMessage(0)
     case win.WM_PAINT:
	      ps := win.PAINTSTRUCT{}
	      hdc := win.BeginPaint(hwnd, &ps)
	     r := ps.RcPaint
	     hrgn := win.CreateRectRgn(r.Left, r.Top, r.Right, r.Bottom)
	     win.FillRgn(hdc, hrgn, win.COLOR_WINDOW+1)
	     win.EndPaint(hwnd, &ps)
     }
     return win.DefWindowProc(hwnd, msg, wParam, lParam)
 }

[TotallyGamerJet]

SOLUTION
Make sure to include the size of the Window Class before registering it
wc.CbSize = uint32(unsafe.Sizeof(wc))