diff --git a/exercise3/wave_1d_parallel.c b/exercise3/wave_1d_parallel.c
index ef9847c..fc2ac7c 100644
--- a/exercise3/wave_1d_parallel.c
+++ b/exercise3/wave_1d_parallel.c
@@ -106,8 +106,12 @@ void time_step(void) {
 // Neumann (reflective) boundary condition.
 void boundary_condition(void) {
   // BEGIN: T6
-  U(-1) = U(1);
-  U(N) = U(N - 2);
+  if (comm_rank == ROOT) {
+    U(-1) = U(1);
+  }
+  if (comm_rank == comm_size - 1) {
+    U(n) = U(n - 2);
+  }
   // END: T6
 }