Transparent windows with AHK what am I doing wrong?

[nite_monkey]

I'm trying to make an AHK script that will let me change how transparent a window is.
This script works, but it randomly decides to mess up.

CapsLock & WheelDown::
{
WinGetActiveTitle, cwindow
WinGet, tp, Transparent, %cwindow%
tp := tp-10
WinSet, Transparent, %tp%, %cwindow%
Return
}

CapsLock & WheelUp::
{
WinGetActiveTitle, cwindow
WinGet, tp, Transparent, %cwindow%
tp := tp+10
WinSet, Transparent, %tp%, %cwindow%
Return
}

Rwin & LWin::
{
WinGetActiveTitle, cwindow
WinSet, Transparent, Off, %cwindow%
Return
}

That is my script.
The problem is sometimes when ever I change active windows and then push the capslock and scroll down, it makes the active window completely transparent.
I just want it to drop the transparency by 10.
Is there a different way I can do this to make it work better?

What I am wanting to achieve is:
When I hold down the capslock and scroll down, decrease the transparency of the active window by 10.
When I hold down the capslock and scroll up, increase the transparency of the active window by 10.

[jgpaiva]

I think your problem is caused by this behaviour:

outputvar is made blank if: 1) the OS is older than Windows XP; 2) there are no matching windows; 3) the window has no transparency level;

-ahk help file (winget section)

In windows with no transparency, the output variable is set to blank, and when you take 10 out of that, you get blank again, which then works as a 0 on the WinSet, setting the window to invisible.
Just add the following to the line after the WinGet:

[Select]

if (tp = "")
  tp =255

That should solve your problem (I hope! )

[nite_monkey]

You know, that might be the problem. I will have to do that. Your explanation would make sense as to why it is doing that.

Edit: I do believe that fixed the problem. I can't get it to mess up on me anymore. Thanks 

[jgpaiva]

Glad to know that!