Every time I try to enter a purchase on the system I get the following error message

Traceback (most recent call last): File "/usr/lib/python2.7/dist-packages/odoo/http.py", line 642, in _handle_exception return super(JsonRequest, self)._handle_exception(exception) File "/usr/lib/python2.7/dist-packages/odoo/http.py", line 684, in dispatch result = self._call_function(**self.params) File "/usr/lib/python2.7/dist-packages/odoo/http.py", line 334, in _call_function return checked_call(self.db, *args, **kwargs) File "/usr/lib/python2.7/dist-packages/odoo/service/model.py", line 101, in wrapper return f(dbname, *args, **kwargs) File "/usr/lib/python2.7/dist-packages/odoo/http.py", line 327, in checked_call result = self.endpoint(*a, **kw) File "/usr/lib/python2.7/dist-packages/odoo/http.py", line 942, in __call__ return self.method(*args, **kw) File "/usr/lib/python2.7/dist-packages/odoo/http.py", line 507, in response_wrap response = f(*args, **kw) File "/usr/lib/python2.7/dist-packages/odoo/addons/web/controllers/main.py", line 895, in call_kw return self._call_kw(model, method, args, kwargs) File "/usr/lib/python2.7/dist-packages/odoo/addons/web/controllers/main.py", line 887, in _call_kw return call_kw(request.env[model], method, args, kwargs) File "/usr/lib/python2.7/dist-packages/odoo/api.py", line 689, in call_kw return call_kw_multi(method, model, args, kwargs) File "/usr/lib/python2.7/dist-packages/odoo/api.py", line 680, in call_kw_multi result = method(recs, *args, **kwargs) File "/usr/lib/python2.7/dist-packages/odoo/models.py", line 5555, in onchange record._onchange_eval(name, field_onchange[name], result) File "/usr/lib/python2.7/dist-packages/odoo/models.py", line 5453, in _onchange_eval method_res = method(self) File "/usr/lib/python2.7/dist-packages/odoo/addons/account/models/account_invoice.py", line 463, in _onchange_invoice_line_ids taxes_grouped = self.get_taxes_values() File "/usr/lib/python2.7/dist-packages/odoo/addons/account/models/account_invoice.py", line 655, in get_taxes_values val = self._prepare_tax_line_vals(line, tax) File "/usr/lib/python2.7/dist-packages/odoo/addons/account/models/account_invoice.py", line 631, in _prepare_tax_line_vals 'base': tax['base'], KeyError: 'base'

The  Value Added Tax amount does not get added even though I've chosen the correct type, i.e. Standard 20%

Please help me to resolve this issue