diff --git a/lectures/opt_transport.md b/lectures/opt_transport.md
index e6d1ab348..30dc9ae5c 100644
--- a/lectures/opt_transport.md
+++ b/lectures/opt_transport.md
@@ -681,7 +681,7 @@ X
 Sure enough, we have the same solution and the same cost
 
 ```{code-cell} ipython3
-total_cost = np.sum(X * C)
+total_cost = np.vdot(X, C)
 total_cost
 ```