c# - Make textbox default value equals to 0 -

i have set textbox = 0

<p>credit balance: </p><asp:textbox id="txtcredit" runat="server" style="width:70px" text = "0"></asp:textbox> 

and code behind this:

scn.open();  sqlcommand cmd = new sqlcommand("select creditrequest userdata username=@username", scn);  cmd.parameters.add("@username", sqldbtype.nvarchar).value = session["new"];  object value = cmd.executescalar();  if (value != null) {     txtcredit.text = convert.todecimal(value).tostring("#,##0.00"); } 

what want happen if has value, display it. if none, default value equal 0. receiving error

object cannot cast dbnull other types. 

i suspect, value coming dbnull, guard against dbnull using convert.isdbnull method.

if (!convert.isdbnull(value)) {     txtcredit.text = convert.todecimal(value).tostring("#,##0.00"); }