Inserted text is vertically flipped

[myvodx]

I followed the sample to insert text to a PDF file

var pt pdft.PDFt
err := pt.Open(pdfsource)
if err != nil {
	panic("Couldn't open pdf.")
}


err = pt.AddFont("arial", "./arial.ttf")
if err != nil {
    t.Error(err)
    return
}

err = pt.SetFont("arial", "", 14)
if err != nil {
    panic(err) 
}

//insert text to pdf
err = pt.Insert("Hello", 1, 10, 10, 100, 100, gopdf.Center|gopdf.Bottom, &pdft.FontColor{R: 255, G: 119, B: 51})
if err != nil {
    panic(err) 
}

However the output of inserted text is vertically flipped (orange)

Does anyone know how to correct it?
Thank you!

[myvodx]

sample.pdf
Here is the output is vertical flipped

sample-output.pdf