Issue getting values with DTP from InfoCube to DSO

I'm loading data from an InfoCube to an DSO.
In the InfoCube I have the following:
DOC_NUM     DOC_ITEM     KNART     VALUE
 
 
In the DSO I need just values whose KNART = MWST.  In transformation I've tried by formula or by routine to do that condition:
Formula: IF(KNART='MWST',VALUE,")
Routine:
IF SOURCE_FIELDS-KNART = 'MWST'.
       RESULT = SOURCE_FIELDS-VALUE.
ENDIF.

But I'm always extracting this: value is not filled.
KNART field is not in DSO

Do you know what I'm doing wrong?

Sol:  End Routine is possible to do it calling directly to active data table for the source infocube.  However I'm not implementing this in my case because the scope of doing it in the direct transformation was to free the End Routine as it's a very complex one and I'm optimizing it to improve the performance.

So, after many tests and tries, I couldn't get the values as it was overwriting all the time to the last value = 0, even if putting sum in the transformation

Sol: You can simply create a Filter in the DTP between Cube to DSO. Keep MWST value in the KNART field in the filter. That's it, you will get the required data in the DSO. No routine is required in the transformation.

Sol:
You need a start routine here and not a field routine.

DELETE SOURCE_PACKAGE WHERE KNART NE 'MWST'.