How to output a string in a derived column if criteria is met.

Hello,

I need to evaluate the following and output the results using a derived column in SSIS.

I have three columns: 

account_number = int
[type] = int
[freq_mode] = varchar(3)

If type = 0, then I want to use the value of freq_mode otherwise the value of the derived column should be NULL.

This is what I have so far, but I am getting a parsing error. 

(DT_STR, 3, 1252)( type == 0 ?  [freq_mode] : NULL (DT_STR, 3, 1252))

Any help or guidance would be appreciated.

Thank you in advance.