diff --git a/src/operators/upwind4.rs b/src/operators/upwind4.rs
index d4a4a22..2dfc9b8 100644
--- a/src/operators/upwind4.rs
+++ b/src/operators/upwind4.rs
@@ -66,7 +66,7 @@ impl Upwind4 {
         let first_elems = prev.slice(s!(..7));
         for (bl, f) in block.outer_iter().zip(&mut fut) {
             let diff = first_elems.dot(&bl);
-            *f = diff * idx;
+            *f += diff * idx;
         }
 
         for (window, f) in prev