diff --git a/ObservationSim/MockObject/SpecDisperser/disperse_c/disperse.pyx b/ObservationSim/MockObject/SpecDisperser/disperse_c/disperse.pyx
index 165450fe1c712ab83918ece7470306d1c3999ee5..f5be0b2794ae24d94c1f17f7e50062bf0f1d26b4 100644
--- a/ObservationSim/MockObject/SpecDisperser/disperse_c/disperse.pyx
+++ b/ObservationSim/MockObject/SpecDisperser/disperse_c/disperse.pyx
@@ -53,7 +53,6 @@ def disperse_grism_object(np.ndarray[FTYPE_t, ndim=2] flam,
 
     nk = len(idxl)
     nl = len(full)
-    yfrac_off = 1-yfrac
 
     if (flat is not None):
         nlamb = len(wlambda)
@@ -92,7 +91,7 @@ def disperse_grism_object(np.ndarray[FTYPE_t, ndim=2] flam,
                     k2 = idxl[k]+(j-1)*shg[1]+i
                     if (k2 >= 0) & (k2 < nl):
                         flat_ids = k2*nlamb+k
-                        full[k2] += fl_ij*yfrac_off[k]*flat_eff_all[flat_ids]
+                        full[k2] += fl_ij*(1-yfrac[k])*flat_eff_all[flat_ids]
 
     else:
         for i in range(0-x0[1], x0[1]):
@@ -110,11 +109,11 @@ def disperse_grism_object(np.ndarray[FTYPE_t, ndim=2] flam,
                 for k in range(nk):
                     k1 = idxl[k]+j*shg[1]+i
                     if (k1 >= 0) & (k1 < nl):
-                        full[k1] += fl_ij*yfrac[k]*ysens[k]
+                        full[k1] += ysens[k]*fl_ij*yfrac[k]
 
                     k2 = idxl[k]+(j-1)*shg[1]+i
                     if (k2 >= 0) & (k2 < nl):
-                        full[k2] += fl_ij*yfrac_off[k]*ysens[k]
+                        full[k2] += ysens[k]*fl_ij*(1-yfrac[k])
 
     return True