Data type mismatch in criteria expression

Question

I am developing a vb.net application with MS Access Database . 

I have a field(cut) with double data type in table marks of markss.mdb .

Following is my code:

        Dim cob As Double

        cob = CDbl(TextBox1.Text)'to convert textbox value to double

        dbprovider = "PROVIDER=Microsoft.Jet.OLEDB.4.0;"

        dbsource = "Data Source=F:\vb\markss.mdb"

        con.ConnectionString = dbprovider & dbsource

        con.Open()

        sql = "select name from marks where cut = 'cob'"

        da = New OleDb.OleDbDataAdapter(sql, con)      

        da.Fill(ds, "result")

On debugging , when I enter ,say 7.9 , in Textbox 1 ,I get an error saying Data type mismatch in criteria expression.

Please help me to resolve this issue , guys ..

Thanks in advance !!!