Smo.Server.ExecuteNonQuery throw an error executing a script with IF statement

Question

I am trying to executed this script (see below) using Smo libraries with powershell (this script works in SSMS)
use master;
GO
use master;
if (not exists(select * from sys.databases where name = 'dbname'))
    alter database [dbname] set offline with rollback immediate;
GO
however, I always got an
Exception calling "ExecuteNonQuery" with "1" argument(s): "An exception occurred while executing a Transact-SQL statement or batch."
if I delete the if statement, the script execute without erros... is there any problem to use if statements?  I would really appreciate any help
regards.!

Answer 1

Olaf
Helper answer does make sense and you won't be able to modify database which doesn't exists. However, your syntax's are correct.
However, if you are executing it via Powershell as mentioned by HoroChan then
I recommend you to increase the statementTimeout to appropriate value. 
There may be chances that alerting database got hanged and exceeded the timeout.  So instead of IMMEDIATE ROLLBACK you can use
WITH NO WAIT
From MSDN:
Specifies that if the requested database state or option change cannot complete immediately without waiting for transactions to commit or roll back on their own, the request will fail.
So your overall code will look like 
USE MASTER;
IF EXISTS(select * from sys.databases where name = 'dbname')
ALTER DATABASE [dbname] SET OFFLINE WITH NO_WAIT
GO
Hope this will help.