Failed to write customized tag into Tiff file #2745

I am using python version 2.7

I am trying to write some custom tag(37000 in this case) into new tiff file. It is able to read tag from image but when I am trying to write them it is not successfully written.

When I am use pillow 4.1.0 to do that, tag is not added, and then when I tried old version pillow 2.3, tag is added successfully but cannot save image correctly (only blank)

Could you give me some help, I appreciate that!

Here is the testing code:

from PIL import Image, TiffImagePlugin, TiffTags

def test_custom_metadata():

    img = Image.open('mytifffile.tif')
    img.tag[37000]='my special tag'
    info = TiffImagePlugin.ImageFileDirectory()
    CustomTagId = 37000

    info[CustomTagId] = 'my special tag'
    print(img.tag)
    Image.DEBUG=True
    TiffImagePlugin.WRITE_LIBTIFF = False # Set to True to see it break.
    img.save('temp2.tif',tiffinfo= img.tag)

    img2 = Image.open('temp2.tif')
    print(img2.tag)
test_custom_metadata()

outputs are:
{256: (1703,), 257: (937,), 258: (8,), 259: (5,), 262: (1,), 296: (1,), 273: (8,), 274: (1,), 339: (1,), 277: (1,), 278: (937,), 279: (1849679,), 37000: ('my special tag',), 284: (1,)}
{256: (1703,), 257: (937,), 258: (8,), 259: (5,), 262: (1,), 296: (1,), 273: (8,), 274: (1,), 339: (1,), 277: (1,), 278: (937,), 279: (1849679,), 284: (1,)}

As you can see, 37000: ('my special tag',) is missing.

I doing this because, my original tiff file have some customized tag(51023), and it is 16 bit. So what I want to do is to rescale it to 8 bit which is able to do and keep that tag(51023) and value as it is.