I have a probelm running inference.py -m demo about ssd_tensorflow_traffic_sign_detection HOT 1 OPEN

The inference section works out just fine:

Inference took 1563.1 ms (0.64 fps)

So I assume my model files should be right. I look into the code in 'model.py' and 'data_prep.py' and I print out some results. It seems that 'x_overlap' and 'y_overlap' will always be 0 except the first time:

'box_a':(180, 170, 241, 191)
'box_b':(179, 171, 240, 191)
'x_overlap':60
'y_overlap':20
'intersection': 1200
'iou':0.9223674096848578
------------------------------
'box_a'::(350, 56, 253, -91)
'box_b':(180, 170, 241, 191)
'x_overlap':0
'y_overlap':0
'intersection':0
'iou':0.0
------------------------------
'box_a':(300, 32, 300, 32)
'box_b':(180, 170, 241, 191)
'x_overlap':0
'y_overlap':0
'intersection':0
'iou':0.0

It seems quite strange to me. How to solve this?
By the way, it works out just fine before I changed the 'NUM_CLASSES' in the setting.py from 3 to 2. Does it matter?

from ssd_tensorflow_traffic_sign_detection.