Wierd error while transfering data from FTP server using SSIS Package?

i am working on SSIS package to recieve .csv files to my SQL DB.
MY FTP server is
ftp.sql.att.com

where i can find lot of csv files but i need to pick up only the before day report file so i am using varaible to check and connect to the previous date file.For that i am using below expression
"//Call_"+
RIGHT("0" + (DT_WSTR, 2)  MONTH(dateadd("d", -1, getdate())), 2)
+RIGHT("0" + (DT_WSTR, 2) DAY(dateadd("d", -2, getdate())), 2)
+(DT_WSTR, 4)  YEAR(dateadd("d", -1, getdate()))+ ".csv"

The output of varaible is     Call_02082014.csv


While executing i am getting below error


[FTP Task] Error: An error occurred with the following error message: "Object reference not set to an instance of an object.".


If with out using varaible if i connect directly to that .csv file SSIS task working properly but when i use varaible i am getting weird error.

Note:I cant run every day ssis package so that is the reason i cfreated varaible so it can check the date and pick up exact file..


Can any one help me to solve this issue please ..