diff --git a/source/f90/seteps.f90 b/source/f90/seteps.f90
index 1d797da0b610d57874c61030b4ed0919d8b9ee7f..464c95a702ea9d7233a04ffda8f9a9f4e398deaf 100644
--- a/source/f90/seteps.f90
+++ b/source/f90/seteps.f90
@@ -62,7 +62,7 @@ subroutine seteps(neps, nos, oscType, osc, epsinf, wn, eps, layers)
           wp = osc(1,k)
           y0 = osc(2,k)
           yp = osc(3,k)
-          addDrude = addDrude - dcmplx(wp**2, (y0 - yp) * wn) / dcmplx(wn2, wn * y0)
+          addDrude = addDrude - dcmplx(wp**2, (yp - y0) * wn) / dcmplx(wn2, wn * y0)
         case default
           write (*,'(A)') '*** Error in oscillator type! ***'
       end select