Formula in transformation rule to transfer only open items

Hi gurus,

I created a transformation to transfer only open items from 0FI_GL_4 datasource to ODS 0FIGL_O02. In transformation rule for datasource field "Item status" I want to specify that only those registers with 'O' value should be transfer to data target.

When I indicate "STATUSPS = 'O'" on formula at field/infobject transformation rule, I obtain "formula element does not match type ....." error message.

Which is the correct syntax for the formule?

Thanks in advance.

Ana