Derived column error

Hello,

Using the foreachloop I loop through excel files to get the filename andthen start loading the data into sql server using oledb...
1- I get the filename
2- Then I strip the filedate from the filename and assign to the ssis-variable called FileDate which is of type string
3- I use this variable in the derived column - It is at this stage that I get the following error

[Derived Column [16]] Error: The "Derived Column" failed because truncation occurred, and the truncation row disposition on "Derived Column.Outputs[Derived Column Output].Columns[FileDate]" specifies failure on truncation. A truncation error occurred on the specified object of the specified component.

note that in the derived column transformation editor, in the Derived column name I put the FileDate text, in expression column I put @[User::FileDate] then datatype column is automatically selected as Unicode string [DT_WSTR] and length column is automatically set to 0

can you see what is wrong please?

Paste your whole expression. Something may be wrong there.