I need help on this

[fellasallah]

TypeError at /computer_list/5/make_payment

coercing to Unicode: need string or buffer, NoneType found

Request Method: POST
Request URL: http://127.0.0.1:8000/computer_list/5/make_payment
Django Version: 1.8
Exception Type: TypeError
Exception Value:

coercing to Unicode: need string or buffer, NoneType found

Exception Location: C:\Users\FELLA\Desktop\FELLA2\src\eyecareapp\views.py in computer_make_payment, line 131
Python Executable: C:\Python27\python.exe
Python Version: 2.7.14
Python Path:

['C:\\Users\\FELLA\\Desktop\\FELLA2\\src',
'C:\\WINDOWS\\SYSTEM32\\python27.zip',
'C:\\Python27\\DLLs',
'C:\\Python27\\lib',
'C:\\Python27\\lib\\plat-win',
'C:\\Python27\\lib\\lib-tk',
'C:\\Python27',
'C:\\Python27\\lib\\site-packages']

Server time: Fri, 18 May 2018 12:13:30 +0000
Traceback Switch to copy-and-paste view

C:\Python27\lib\site-packages\django\core\handlers\base.py in get_response

response = wrapped_callback(request, *callback_args, **callback_kwargs)

...
▶ Local vars
C:\Users\FELLA\Desktop\FELLA2\src\eyecareapp\views.py in computer_make_payment

instance.paid = instance.paid + instance.deposit