validation: 'expression does not produce a value'

Hi, I faced a small problem here.. in the validation part.
Here goes the code:


If (tb_username.Text = "") Then Response.Write("<script language=javascript>alert('Please enter username.')</script>") ElseIf (tb_email.Text = "") Then Response.Write("<script language=javascript>alert('Please enter email.')</script>") ElseIf (tb_pwd.Text = "") Then Response.Write("<script language=javascript>alert('Please enter password.')</script>") ElseIf (ddl_email.SelectedValue = "") Then Response.Write("<script language=javascript>alert('Your email is not complete.')</script>") ElseIf (ddl_qns.SelectedValue = "") Then Response.Write("<script language=javascript>alert ...