Accessing effective stress variables when using axisymetric option in FISH

Question

I'm running a function defined in FISH in an axisymetric FLAC model. The model accesses the stresses in the three different axes using sxx, syy and szz. I've used this function many times in a plain strain model without any issues but in the axisymetric model the values of sxx, syy and szz appear to be returning 0. I suspect that this is some sort of standard behaviour for FLAC but I can't find a reference to it in the documentation. Does anyone know if this is the case, and if it is how to access the values of sxx, syy and szz for an axisymetric analysis. For reference the code used to access the variables is shown below:

def store_p
loop i (1,izones)
  loop j (1,zones)
    ex_3(i,j) = -(sxx(i,j) + syy(i,j) + szz(i,j)) / 3.0 - pp(i,j)
    ex_15(i,j) = sxx(i,j)
    ex_16(i,j) = syy(i,j)
    ex_17(i,j) = szz(i,j)
  end_loop
end_loop
end