sql server 2005 ssis error when creating a derived column

Hi,
    I am creating a package in SSIS that imports data into a sql server 2005 database table from a csv file.
The csv file has columns, first name, last name and I want to create a derived column  called Full_ name.
The expression I am using is SET Full_Name = first name +' ' + last name
which works fine in t-sql  but I am getting the following error in ssis..

Any help appreciated. Thanks

TITLE: Microsoft Visual Studio
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Error at Data Flow Task [Derived Column [115]]: Attempt to parse the expression "SET Full_Name= first name+' ' +last name" failed. The expression might contain an invalid token, an incomplete token, or an invalid element. It might not be well-formed, or might be missing part of a required element such as a parenthesis.

Error at Data Flow Task [Derived Column [115]]: Cannot parse the expression "SET Full_Name= first name+' ' +last name". The expression was not valid, or there is an out-of-memory error.

Error at Data Flow Task [Derived Column [115]]: The expression "SET Full_Name= first name+' ' +last name" on "output column "Full_Name" (263)" is not valid.

Error at Data Flow Task [Derived Column [115]]: Failed to set property "Expression" on "output column "Full_Name" (263)".



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ADDITIONAL INFORMATION:

Exception from HRESULT: 0xC0204006 (Microsoft.SqlServer.DTSPipelineWrap)

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BUTTONS:

OK