Customisable-MsgBox doesn't return -1 with no selection

I was trying out the Customisable MessageBox v0.98 beta-3 and noticed behavior that doesn't seem to conform to the documentation.

My understanding is that if no selection is made, then $0 will be set to -1. The code below ends up with $0 being 3 if the ESC button is pressed, or the MessageBox window is closed with the upper right X.

Am I doing something wrong, or leaving something out?

Name "Customisable Messagebox plug-in test"



OutFile "Example.exe"

ShowInstDetails show



Section

   # show four buttons

   messagebox::show MB_DEFBUTTON1 "Test 1" \

   "0,103" \   

   "Text that will show up" \

   IDYES IDNO "Maybe" "Who Cares"



   Pop $0

   # Put $0 in $R1 just in case something changes $0

   # Earlier testing used $0 with same results

   StrCpy $R1 $0

   DetailPrint "messagebox call 1 returned... $0"

   

   StrCmp $R1 1 0 +3

   DetailPrint "Yes selected"

   Goto done

   StrCmp $R1 2 0 +3

   DetailPrint "No selected"

   Goto done

   StrCmp $R1 3 0 +3

   DetailPrint "Maybe selected"

   Goto done

   StrCmp $R1 4 0 done

   DetailPrint "Who Cares selected"

   Goto done



   done:

SectionEnd