streaming mean iou

[myBestLove]

first thanks for your work
and I found you use streaming mean iou as evaluation method.
but when I output the result, I found the first result is zeros. like:
step 0 , mean_iou: 0.000000 (4.134 sec/step) setp 1 , mean_iou: 0.324524(0.286 sec/step)

I don't know why the result is zeros even my pred_img and gt_img is the same? I search many research and streaming_mean_iou in tensorflow is calculated through (tp/(tp+fp+fn)), I really want to know the reason? thanks for answer.