How to cast empty string

I want to replace a column value with a null if the string is empty. I would have thought this simple expression would do it:

RTRIM([FromContractSymbol]) == "" ? NULL(DT_STR, 0, 1252) : (DT_STR, 6, 1252)FromContractSymbol

Yet, I get the following error:

For operands of the conditional operator, the data type DT_STR is supported only for input columns and cast operators. The expression "...see above..." has a DT_STR operand that is not an input column or the result of a cast, and cannot be used with the conditional operation.

The expression works if I replace the NULL(DT_STR, 0, 1252) with say "A" and the expression works on other non-string columns. (As in "NULL(DT_I1) : (DT_I1)100")

The error does explain how to solve it. Although you have specified the type for the NULL you have to cast it.

So if you change your line to

RTRIM([FromContractSymbol]) == "" ? (DT_STR, 6, 1252)NULL(DT_STR, 6, 1252) : (DT_STR, 6, 1252)FromContractSymbol

It should work