TypeError: '>=' not supported between instances of 'NoneType' and 'float'

问题遇到的现象和发生背景

在服务器用python3.7跑代码时报错

问题相关代码，请勿粘贴截图

def own_loader(id, root):  # 这里只导入了原始数据的，用的就是这个
    img = cv2.imread(os.path.join(root, '{}_sat.png').format(id))
    mask = cv2.imread(os.path.join(root + '{}_mask.png').format(id), cv2.IMREAD_GRAYSCALE)
    mask = np.expand_dims(mask, axis=2)
    # img = np.array(img, np.float32).transpose(2,0,1)/255.0 * 3.2 - 1.6
    # mask = np.array(mask, np.float32).transpose(2,0,1)/255.0
    mask[mask >= 0.5] = 1
    mask[mask <= 0.5] = 0
    img = np.array(img, np.float32).transpose(2, 0, 1)
    mask = np.array(mask, np.float32).transpose(2, 0, 1)
    return img, mask

class ImageFolder(data.Dataset):

    def __init__(self, trainlist, root):
        self.ids = trainlist
        # self.loader = default_loader  #原始的
        self.loader = own_loader  # 用了自己的数据导入
        self.root = root
        # self.trans = transforms.Compose([transforms.ToTensor()])
        # self.trans = transforms.Compose([transforms.ToTensor(),transforms.Normalize(mean=(0.5,0.5,0.5),std=(0.5,0.5,0.5))])

    def __getitem__(self, index):
        id = list(self.ids)[index]
        img, mask = self.loader(id, self.root)
        # img = np.transpose(img, (1,2,0))
        # img = self.trans(img)
        img = torch.Tensor(img)
        mask = torch.Tensor(mask)
        return img, mask

    def __len__(self):
        return len(list(self.ids))

运行结果及报错内容

line 132, in own_loader
mask[mask >= 0.5] = 1
TypeError: '>=' not supported between instances of 'NoneType' and 'float'

已解决，标签图没有读进去，数据集的路径出错，一定要检查好图片路径！

mask[mask >= 0.5]是什么语法
mask到底是个列表还是数字啊，怎么括号外面把它当列表用，括号里面又把它当数字用？
此外，报错信息是说mask的值是None