Cpaturing a window using ximagesrc gives black borders around the image

I was trying to capture a single window using ximagesrc. Encoding it to png and saving it gives black bordes around the image. This was the command i used.

gst-launch-1.0 ximagesrc remote=1 num-buffers=1 xid=<windowid> ! videoconvert ! pngenc ! filesink location=screenshot.png

and this is the result I've got

is there any way to remove that borders or to decrease border's alpha?