Silent installation of SQL 2014 express edition is not working with the code used for SQL 2005 express edition

My application installer does the silent installation of SQL Express 2005. I am trying with the same code to install SQL Express 2014. Here is the code that i am using.

               string SQLfn = strSQLPath;
                string SQLp = "/qn INSTANCENAME=SQLEXPRESS ADDLOCAL=ALL";
                ProcessStartInfo psiSQL = new ProcessStartInfo(SQLfn, SQLp);
                Process SQLprocess = new Process();
                try
                {
                    SQLprocess = System.Diagnostics.Process.Start(psiSQL);
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }

I am getting Error code: 0x84B40001.

Error Message: The syntax of argument "/QN" is incorrect. Either the delimiter '=' is mising or there is one or more space characters before delimiter'='. Please use /? to check usage.

Cases i tried:
1. SQL Express 2005 i am not getting this error.
2. SQL Express 2014 normal installation is working fine as well. So only silent installation is creating problem.
3. I have SQL Server 2008 in my machine with instance name SQLSERVER.

I browsed through net and found /QN is one of the command used for silent installation with no UI.

But this error makes me strange.

Mine is a development machine. I have different visual studio versions in machine and SQL Server 2008 R2.

And the reason i stick to code, i successfully ran the same code with SQLExpress 2014 an year (but things got erased from my mind as well.)

Please do suggest, what could be the work around to fix this issue.

Thanks for your support in advance.

Regards, Siva.