title: Error while Register Payment in customer invoice odoo
body: <p>I didn't understand why this happen because error shows that object or function is not iterable and this error comes from base files not from any custom modules, error shows that it's pythonic.</p>
  
  <p>Here is the error traceback.</p>
  
  <pre><code>Traceback (most recent call last):
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/http.py", line 537, in _handle_exception
      return super(JsonRequest, self)._handle_exception(exception)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/http.py", line 574, in dispatch
      result = self._call_function(**self.params)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/http.py", line 310, in _call_function
      return checked_call(self.db, *args, **kwargs)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/service/model.py", line 118, in wrapper
      return f(dbname, *args, **kwargs)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/http.py", line 307, in checked_call
      return self.endpoint(*a, **kw)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/http.py", line 803, in __call__
      return self.method(*args, **kw)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/http.py", line 403, in response_wrap
      response = f(*args, **kw)
    File "/home/viraj/workspace/pansuriya/odoo_v8/addons/web/controllers/main.py", line 944, in call_kw
      return self._call_kw(model, method, args, kwargs)
    File "/home/viraj/workspace/pansuriya/odoo_v8/addons/web/controllers/main.py", line 936, in _call_kw
      return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/api.py", line 250, in wrapper
      return old_api(self, *args, **kwargs)
    File "/home/viraj/workspace/pansuriya/odoo_v8/addons/account_check_writing/account_voucher.py", line 93, in create
      return super(account_voucher, self).create(cr, uid, vals, context=context)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/api.py", line 250, in wrapper
      return old_api(self, *args, **kwargs)
    File "/home/viraj/workspace/pansuriya/odoo_v8/addons/mail/mail_thread.py", line 381, in create
      thread_id = super(mail_thread, self).create(cr, uid, values, context=context)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/api.py", line 250, in wrapper
      return old_api(self, *args, **kwargs)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/api.py", line 345, in old_api
      result = method(recs, *args, **kwargs)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/models.py", line 4092, in create
      record = self.browse(self._create(old_vals))
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/api.py", line 248, in wrapper
      return new_api(self, *args, **kwargs)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/api.py", line 481, in new_api
      result = method(self._model, cr, uid, *args, **kwargs)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/models.py", line 4191, in _create
      updates.append((field, '%s', current_field._symbol_set[1](vals[field])))
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/osv/fields.py", line 409, in &lt;lambda&gt;
      self._symbol_f = lambda x: _symbol_set_float(self, x)
    File "/home/viraj/workspace/pansuriya/odoo_v8/openerp/osv/fields.py", line 386, in _symbol_set_float
      precision, scale = digits
  TypeError: 'function' object is not iterable
  </code></pre>
  
  <p>According to code, <strong>button_proforma_voucher</strong> method of <strong>account_voucher</strong> model should be called but control does comes upto that and error raised before that.</p>