Error - Value cannot be null.

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C#

Value cannot be null.
Parameter name: dataSet
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentNullException: Value cannot be null.
Parameter name: dataSet

Source Error:

Line 672:        da.Fill(ds,"cv");<br />
Line 673:        da1=new OleDbDataAdapter(sql,con);<br />
Line 674:        da1.Fill(ds1,"cv");<br />
Line 675:		int r=0,r1=0;<br />
Line 676:		DataRow []rs;

My code is given below

s1 = TextBox1.Text.Trim();
        s2 = TextBox2.Text.Trim();
      
        con.Open();
       	string sql = "select * from cv";
       // string sql1 = "select * from cv";
        da=new OleDbDataAdapter(sql,con);
        da.Fill(ds,"cv");
        da1=new OleDbDataAdapter(sql,con);
        da1.Fill(ds1,"cv");
		int r=0,r1=0;
		DataRow []rs;
		DataRow []rs1;
		rs=ds.Tables["cv"].Select("email='" + s1 + "'");
		r=rs.Length;
		rs1=ds1.Tables["cv"].Select("pass='" + s2 + "'");
		r1=rs1.Length;
		   try {       
            if (s1 == "" && s2 == "")
			{
                Label1.Text = "Please enter your Email ID and Password";
			}
            else 
			{
			if (r>0)
			{
			if(r1>0)
			{
                Response.Redirect("account.aspx");
			}
			else
			{
                Label1.Text = "Email ID not registred or Check your Password";
			}
			}
			}
			}
			catch (Exception ex)
			{
			Response.Write(ex.Message);
			}
          con.Close();