Image glitch using golang's drawmask with draw.Over

I am rewriting with golang a script I originally wrote in python. The function I am trying to recreate currently takes one image (a coloured overlay) and pastes it on a background (a map) of the same size, with 50% transparency. The desired output, as produced by my python script, is:

The golang code I have written to try to replicate this in golang, with reference to this stackoverflow thread.

In main.go

// overlayImg is type image.Image, consists of the coloured overlay
overlayImg = lib.AddMap(overlayImg)
// lib.SaveToRecords is a function that saves image.Image overlayImg to path specified by string download.FileName
lib.SaveToRecords(overlayImg, download.FileName)

In package myproject/lib

func AddMap(i image.Image) image.Image{
    // mapFile is the background map image
    mapFile, err := os.Open("./res/map.png")
    if err != nil { panic(err) }
    mapImg, _, err := image.Decode(mapFile)
    if err != nil { panic(err) }
    mask := image.NewUniform(color.Alpha{128})

    canvas := image.NewRGBA(mapImg.Bounds())
    draw.Draw(canvas, canvas.Bounds(), mapImg, image.Point{0, 0}, draw.Src)

    draw.DrawMask(canvas, canvas.Bounds(), i, image.Point{0, 0}, mask, image.Point{0, 0}, draw.Over)

    return canvas
}

However, the resulting image is produced with a sort of 'glitch' in parts of the coloured overlay. This colour glitch is reliably reproduced on each run of the script, even with different overlays (the map stays the same throughout, of course).

How do I get rid of the 'glitch'?

Things I have tried:

• Using draw.Draw in place of draw.DrawMask with draw.Over as the 'op' setting. Results in same colour 'glitch', but without transparency.

• Using draw.DrawMask, except with draw.Src as the 'op' setting. Result:

• Using draw.Draw in place of draw.DrawMask, and with draw.Src as the 'op' setting. This was with a slightly different overlay image. Result:

Update

I've tried assigning mask := image.NewUniform(color.Alpha16{32767}) in place of mask := image.NewUniform(color.Alpha{128}) according to putu's comment. Glitch still appears. Additionally, this glitch is not as consistent as I thought, showing up at only roughly 10% of the time. It seems the glitch shows up depending on the content of the image pasted unto the background.

Update 2

Originally, mapImg was of type *image.NRGBA, i and canvas of type *image.RGBA. Once again following the advice in the comments, I converted mapImg to type *image.RGBA to match with the rest. To do this, I used the following few lines of code:

mapImg, _, err := image.Decode(mapFile)
mapImgRGBA := image.NewRGBA(mapImg.Bounds())
draw.Draw(mapImgRGBA, mapImgRGBA.Bounds(), mapImg, image.Point{0, 0}, draw.Src)
// Use mapImgRGBA in place of mapImg from here

Still don't work:

Thanks to use noethics in the freenode #go-nuts channel for requesting a look at the original overlay, I found the issue while generating the output after each line that manipulates the image.

The issue is not with "image/draw", but with an external library I'm using to resize the overlay to the dimensions of the map, "github.com/nfnt/resize".

The overlay before resizing:

The overlay after resizing (red & black pixels undesirable):

Update

It's not even an issue with "github.com/nfnt/resize". I was simply using the wrong interpolation algorithm. Turns out bilinear gives me the best output for the image I am working with. I suspect it's also the one used in the library I used to write my original script in.