Using Tparam to pass parameters to an SQL statement

I am new to Delphi and FireDAC.  Could anybody answer why the following code generates an err at runtime:

  //  Change state name procedure

TFRM_States.btnChangeStateClick(Sender: TObject);

var  SQL: string; 

SQLParams: TParams;

begin  lbStateActions.Items.Add( 'Updated: ' + ed_StateCode.Text ); 

SQLParams.CreateParam( ftString, 'StateCode', ptInput ); 

SQLParams.CreateParam( ftString, 'StateName', ptInput ); 

SQLParams.ParamByName( 'StateCode' ).AsString := QuotedStr( ed_StateCode.Text ); 

SQLParams.ParamByName( 'StateName' ).AsString := QuotedStr( ed_StateName.Text ); 

SQL := 'UPDATE STATES SET NAME = :StateName WHERE STATE = :StateCode'; 

DM_ClientDB.FDC_ClientDB.ExecSQL( SQL ); 

RefreshDataset( DM_ClientDB.DS_ClientDB.DataSet );

end;