WinMinimize PID after MsgBox - autohotkey

Question

The following code opens notepad, then minimizes it. It works fine:
!^m::
Run, Notepad.exe,,USEERRORLEVEL MAX, PID
WinWait, ahk_pid %pid%
WinMinimize, ahk_pid %PID%
return
However if I add an MsgBox to display the PID before minimizing the notepad window, then the latter doesn't get minimized:
!^m::
Run, Notepad.exe,,USEERRORLEVEL MAX, PID
WinWait, ahk_pid %pid%
MsgBox OK %PID%
WinMinimize, ahk_pid %PID%
return
Why?
Sleeping didn't help:
!^m::
Run, Notepad.exe,,USEERRORLEVEL MAX, PID
WinWait, ahk_pid %pid%
MsgBox OK %PID%
Sleep, 2000 ; 2 seconds
WinMinimize, ahk_pid %PID%
return
I use Windows 7 SP1 x64 Ultimate and AHK 1.1.13.01 (October 11, 2013). I have the same issue with Version v1.0.48.05 and Version v1.1.15.02. All AHK versions I tried are 32-bit.
Edit:
!^b::WinMinimize, ahk_pid 9156 doesn't even work. (9156 being a valid PID)
Following MCL suggestion, I also unsuccessfully tried:
!^t::
PID = 11692
hwnd := WinExist("ahk_pid " pid)
WinMinimize, ahk_id %hwnd%
return

Answer 1

Use this code (it is tested and is working):
!^m::
Run, Notepad.exe,,MAX, PID
CheckWin:
IfWinExist, Untitled - Notepad
{
WinWait, Untitled - Notepad
MsgBox OK %PID%
WinMinimize, Untitled - Notepad
}
else
{
Goto, CheckWin
}
return
WinWait and WinMinimize commands cannot use Process ID (PID). In my code I am using WinTitle with these commands.
I am using IfWinExist command to be sure that notepad.exe is already launched and only after use WinWait and WinMinimize commands.