Replace "NA" or any other values with NULL in derived column SSIS

Hi Guys,
To Replace "NA" or any other values with NULL in derived column SSIS use the below code

ColumnName == "" ? NULL(DT_WSTR,50) : ColumnName

I noticed that the below code did not work

ColumnName == "" ? NULL(DT_STR,50,1252) : ColumnName. So I just used DT_WSTR as it did not make much difference