I have this datatable result in c#

```
Date    Employee Job1   Job2   Job3
1/1/2012    a    1      1      1 
1/1/2012    b           2      
1/1/2012    c    2      1      4
1/1/2012    d    4      2      1
1/2/2012    a    3      2      5
1/2/2012    b    2      2      2
1/2/2012    c    3      3      3
1/2/2012    d    1      1      1
1/3/2012    a    5      5      5
1/3/2012    b    2      2      6
1/3/2012    c           1      1
1/3/2012    d    2      3      4
2/1/2012    a    2      2      2
2/1/2012    b    5      5      2
2/1/2012    c    2      2      2
2/2/2012    a           3      
2/2/2012    b    2      3      3
3/1/2012    a    4      4      2

```

To get this result:

Job1:

```
Employee      January       February            March
A             9             5                   4
B             6             7
C             6             2
D             7

```

The Linq code is:

```
            var monthEmpGroups = tblEmpJobs.AsEnumerable()
                .Select(r => new
                {
                    Row = r,
                    Employee = r.Field<String>("Employee"),
                    Year = r.Field<DateTime>("Date").Year,
                    Month = r.Field<DateTime>("Date").Month
                })
                .GroupBy(x => x.Employee);

            DataTable tblMonthResultJob1 = new DataTable();
            tblMonthResultJob1.Columns.Add("Employee", typeof(string));
            var dtf = System.Globalization.CultureInfo.CurrentCulture.DateTimeFormat;

            foreach (var empGroup in monthEmpGroups)
            {
                string employee = empGroup.Key;
                var newRow = tblMonthResultJob1.Rows.Add();
                newRow["Employee"] = employee;
                var empMonthGroup = empGroup.GroupBy(mg => new { mg.Year, mg.Month });

                foreach (var empYearMonthGroup in empMonthGroup)
                {
                    int year = empYearMonthGroup.Key.Year;
                    int month = empYearMonthGroup.Key.Month;
                    string colName = string.Format("{0} {1}", dtf.GetMonthName(month), year);
                    if (!tblMonthResultJob1.Columns.Contains(colName))
                        tblMonthResultJob1.Columns.Add(colName, typeof(int));
                    int empJob1Count = empYearMonthGroup.Sum(x => x.Row.Field<int>("Job1"));
                    newRow[colName] = empJob1Count;
                }
            }

```

On this line:

```
int empJob1Count = empYearMonthGroup.Sum(x => x.Row.Field<int>("Job1"));
I am getting error: {System.InvalidCastException: Cannot cast DBNull.Value to type 'System.int'. Please use a nullable type. 

```

Can anybody suggest how to resolve this issue.