Input string was not in a correct format (at the time of add values in Grid view)

I want to add items in PO.For that I am using Data Grideview . When user click on add button all the related fields will add in Data Grideview. like that required items will place in PO.I am using following code
 

Dim dr As DataRow
        dr = dt.NewRow()
        dr(0) = ItemInvoiceNo
        dr(1) = obj.ReturnString("Select Part_no From Item_Master Where Item_no=" & cmbPartNo.SelectedValue & "")
        dr(2) = Convert.ToString(txtItemType.Text)
        dr(3) = Convert.ToString(txtItemDesc.Text)
        dr(4) = Convert.ToInt32(txtQty.Text)
        dr(5) = Convert.ToInt32(txtRate.Text)
        dr(6) = Convert.ToInt32(txtDisc.Text)
        dr(7) = Convert.ToInt32(txtAmt.Text)
        dt.Rows.Add(dr)
        GridItem.DataSource = dt.DefaultView
        ItemInvoiceNo = ItemInvoiceNo + 1

 
table structure is as follows:
PO Table
Item_PO_No
PO NO
Item No
Unit
Qty
Rate
Disc
Amt
 
Item Master
Item No
Desc
Part No
Unit
Vat
Item Type code


System give an error after dr(2) = Convert.ToString(txtItemType.Text)

Error:Input string was not in a correct format.Couldn't store in Item_type_code
Column. Expected type is Int64.
I am unable to find exact problem.Please help me.