screenshot window mode

i try to get a sceenshot in window mode, but i get always a blank image. Also the option -t doesn’t work, i get always png. Whats wrong?

"Shot " & (current date) & ".png"
do shell script "screencapture -wt jpg ~/Desktop/" & quoted form of result


switches which require parameters have to be specified separately

property imageFormat : "jpg"
"Shot " & (current date) & "." & imageFormat
do shell script "screencapture -w -t " & imageFormat & " ~/Desktop/" & quoted form of result

Hi and thanks Stefan, that works.
in previous version i could choose some window (one from multiple open windows like in Finder) to shoot. Why doesnt work anymore ?