diff --git a/src/shared/cvmix_math.F90 b/src/shared/cvmix_math.F90
index cdf6fcdca..64d6a302b 100644
--- a/src/shared/cvmix_math.F90
+++ b/src/shared/cvmix_math.F90
@@ -244,7 +244,7 @@ function cvmix_math_evaluate_cubic(coeffs, x_in, fprime)
       cvmix_math_evaluate_cubic = cvmix_math_evaluate_cubic +                 &
                                   coeffs(i)*(x_in**(i-1))
       if (present(fprime).and.(i.gt.2)) &
-        fprime = fprime + real(i-1,cvmix_r8)*(x_in**(i-2))
+        fprime = fprime + coeffs(i)*real(i-1,cvmix_r8)*(x_in**(i-2))
     end do
 
   end function cvmix_math_evaluate_cubic