nested if

I'm sorry, but I have a new problem.

Apparently, it is a problem with a nested if.

If I run the following source with line "Check_Position(30, 60)" , it runs normally, and the window displays correctly the title "Case 1"

But if I run with the line "Check_Position(100, 100)", I have in the black window the mention :

VM crashes on 24931. I do not understand why.

In this case, in the sub-program Check_Position, the condition "x>15 and x<58" of the main IF is false, and normally the execution leaves the sub-program. But it seems that End Sub is not executed.

Or I do not understand how to use if condition statement. Is ELSE mandatory ? Please, can you explain me ?

Sub Check_Position(x, y)
if x>15 and x<58 then

if y>39 and y<81 then
SetWindowTitle("case 1")
elseif y>87 and y<130 then
SetWindowTitle("case 2")
elseif y>135 and y<178 then
SetWindowTitle("case 3")
elseif y>185 and y<226 then
SetWindowTitle("case 4")
End If

End If
End Sub

WindowOpen ( 0,"my window", 198, 114, 285, 270,0)
Window ( 0)
ScreenOpen ( 0, 285, 270, 0, 0, 285,270, 0)
Screen ( 0)

While Inkey()<>27
       Update()
Wait(10)
       ' Check_Position(30, 60)
       Check_Position(100, 100)
Wend