Setting a NULL value in SSIS derived column

Ran into an interesting error while trying to set a null value to a column in a derived column task in SSIS.
Here is my statement:

[CustNum]=="" ? NULL(DT_STR, 4, 1252) : [CustNum]

And the error thrown:

For operands of the conditional operator, the data type DT_STR is supported only for input and cast operations.

The correct syntax for this is:

[CustNum]=="" ? (DT_STR, 4, 1252)NULL(DT_STR, 4, 1252) : [CustNum]

Not sure why this requires the explicit cast since you include the type in the null statement.

peace

 

 

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