Hi everyone i am using odoo 14 CE i have made a custom module in which there is a selection field items with options 
item = fields.Selection([('a', 'LAB Equipment’s'),
('b', 'Software Tools'),
('c', 'External Labs']),)

i am trying to inherit into that using another custom module 

item = fields.Selection(selection_add=[('mech', 'Mechanical'),
                                       ('rework', 'Rework')])

it works but everytime i restart my odoo server crashes and give me error 
Internal server error
The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application

How do i fix this using python code or xml how can i achieve this