Child table Calculation issues

I tried to make calculation in the child table, it’s working in the first row. While making the second row it showing the following error.
Code:

frappe.ui.form.on("Sales Contract", {
  before_save: function (frm) {
var total = 0;
$.each(frm.doc.cost_sheet || [], function (i, d) {
  if (d.based_on == "Per Unit") {
    total = d.quantity * d.cost;
  }        
    total = total.toFixed(2); // Set recurring decimal limit
    d.cost_amt = total;
    d.cost_mt = d.cost_amt / d.quantity;
});
  cur_frm.refresh_field("cost_sheet");
  }
});

Error:
Traceback (most recent call last):
File “/home/frappe/frappe-bench/apps/frappe/frappe/desk/form/save.py”, line 22, in savedocs
doc.save()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 260, in save
return self._save(*args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 312, in _save
self.update_children()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 330, in update_children
self.update_child_table(df.fieldname, df)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 339, in update_child_table
d.db_update()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/base_document.py”, line 335, in db_update
self.db_insert()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/base_document.py”, line 312, in db_insert
), list(d.values()))
File “/home/frappe/frappe-bench/apps/frappe/frappe/database.py”, line 199, in sql
self._cursor.execute(query, values)
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/cursors.py”, line 170, in execute
result = self._query(query)
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/cursors.py”, line 328, in _query
conn.query(q)
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/connections.py”, line 517, in query
self._affected_rows = self._read_query_result(unbuffered=unbuffered)
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/connections.py”, line 732, in _read_query_result
result.read()
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/connections.py”, line 1075, in read
first_packet = self.connection._read_packet()
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/connections.py”, line 684, in _read_packet
packet.check_error()
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/protocol.py”, line 220, in check_error
err.raise_mysql_exception(self._data)
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/err.py”, line 109, in raise_mysql_exception
raise errorclass(errno, errval)
InternalError: (1054, u"Unknown column ‘nan’ in ‘field list’")

Traceback (most recent call last):
File “/home/frappe/frappe-bench/apps/frappe/frappe/app.py”, line 61, in application
response = frappe.handler.handle()
File “/home/frappe/frappe-bench/apps/frappe/frappe/handler.py”, line 21, in handle
data = execute_cmd(cmd)
File “/home/frappe/frappe-bench/apps/frappe/frappe/handler.py”, line 56, in execute_cmd
return frappe.call(method, **frappe.form_dict)
File “/home/frappe/frappe-bench/apps/frappe/frappe/init.py”, line 1036, in call
return fn(*args, **newargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/desk/form/save.py”, line 22, in savedocs
doc.save()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 260, in save
return self._save(*args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 312, in _save
self.update_children()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 330, in update_children
self.update_child_table(df.fieldname, df)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py”, line 339, in update_child_table
d.db_update()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/base_document.py”, line 335, in db_update
self.db_insert()
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/base_document.py”, line 312, in db_insert
), list(d.values()))
File “/home/frappe/frappe-bench/apps/frappe/frappe/database.py”, line 199, in sql
self._cursor.execute(query, values)
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/cursors.py”, line 170, in execute
result = self._query(query)
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/cursors.py”, line 328, in _query
conn.query(q)
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/connections.py”, line 517, in query
self._affected_rows = self._read_query_result(unbuffered=unbuffered)
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/connections.py”, line 732, in _read_query_result
result.read()
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/connections.py”, line 1075, in read
first_packet = self.connection._read_packet()
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/connections.py”, line 684, in _read_packet
packet.check_error()
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/protocol.py”, line 220, in check_error
err.raise_mysql_exception(self._data)
File “/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/pymysql/err.py”, line 109, in raise_mysql_exception
raise errorclass(errno, errval)
InternalError: (1054, u"Unknown column ‘nan’ in ‘field list’")