a question of oil source setup in SA module

Dear All:
      Recently I use the Mike21(2012 Edition, sp2) SA module to simulate the oil spill process. I set one fixed soure,and made a dsf0 file of source strength. But when I load the dsf0 file, it can't be loaded, with a warnning "no valid item".  So I try to change the item of the dsf0 fie to "Undefined, m^3/s", "Volume flux, m^3/s", "Discharge, m^3/s", and neither of them work. But when I set the item as "Mass flux, kg/s", it can be loaded successfully. But, according to the manual, the source strengh should be Volume flux with unit of m^3/s. Is this a bug or something? If I set the item as "Mass flux, kg/s", and run the .npa, will the result still be correct?
    Thanks in advance!