typeError:object of type 'context' has no len()

[linhong1909936]

because I do not have gpu, then I changed the code in 'trian_yolo,py'

training contexts

#ctx = [mx.gpu(int(i)) for i in args.gpus.split(',') if i.strip()]
#ctx = ctx if ctx else [mx.cpu()]
ctx = mx.cpu()

Then I got the error:
File "C:\Users\Chinese\anaconda3\envs\shMGO\lib\site-packages\mxnet\gluon\utils.py", line 110, in split_and_load
if len(ctx_list) == 1:

TypeError: object of type 'Context' has no len()

please help
Thanks