There are 1-pixel dark edges on the image after using "imwarp"

I try to do image stitching, and now I am testing the function "imwarp":

 % code
panoramaView = imref2d([400 400], [-200 200], [-200 200]);
warpedImage = imwarp(I, tforms,'OutputView',panoramaView);

to transform the image with a given tforms, while after transformation the image always has 1-pixel dark edges. The original image is:

After using "imwarp"：

1)After transformation：

2)zoom in the up right corner to see the 1-pixel dark edge:

Even I tried with tforms.T=[1 0 0; 0 1 0; 0 0 1], it still has this edge. And with this edge I cannot obtain a good image stitching result. The dark edge will show in the panorama.

Can anyone help me to figure out this? Thank you very much.