Drawing function query

I made this "Dibujo(guibank)" function for use in a paint program I am making. My question is to why it won't work.
Please only explain what is wrong, not how to fix it... because
it will be a vital part in a program I'm making for the Blitz Showcase, and I want it to be ENTIRELY my coding.
Thanks,
Siopses

;GUI

;Windows
window=CreateWindow("MICHAELSOFT DIBUJO",300,400,555,555,window,15)

;Canvases
canvas=CreateCanvas(0,0,430,450,window)

;Menu
menu=WindowMenu(window)
File=CreateMenu("FILE",1,menu)
CreateMenu("END",2,File)
CreateMenu("Clear",3,File)

UpdateWindowMenu window

;Buttons
draw=CreateButton("DRAW",450,150,50,25,window,2)

canvasx=430
canvasy=450
guibank=CreateBank(24)
PokeInt guibank,0,window
PokeInt guibank,4,canvas
PokeInt guibank,8,File
PokeInt guibank,12,draw
PokeInt guibank,16,canvasx
PokeInt guibank,20,canvasy

.begin

Repeat
	WaitEvent()
	Dibujo(guibank)
Until EventID()=$803
AppTitle("Quit?")
blah=Proceed("Are you sure"+Chr(10)+"you want to quit?")
	If blah=1 Then
		End
			Else
				If blah<>1 Then
					Goto begin
				EndIf
	EndIf

Function Dibujo(guibank)
	canvas=PeekInt(guibank,4)
	File=PeekInt(guibank,8)
	draw=PeekInt(guibank,12)
	canvasx=PeekInt(guibank,16)
	canvasy=PeekInt(guibank,20)
		SetBuffer CanvasBuffer(canvas)
			Select EventID()
				Case $203
					If MouseX>canvasx 
						If MouseY>canvasy
							If ButtonState(draw)=True Then
								Plot MouseX,MouseY
									FlipCanvas canvas
							EndIf
						EndIf 
					EndIf 
			End Select
End Function