diff --git a/src/main/scala/problems/Mux4.scala b/src/main/scala/problems/Mux4.scala
index 3638c5ef..3ee5f5ce 100644
--- a/src/main/scala/problems/Mux4.scala
+++ b/src/main/scala/problems/Mux4.scala
@@ -41,5 +41,8 @@ class Mux4 extends Module {
   //Implement below ----------
 
 
+
+  // make the compile process happy, needs to be substituted by the output of the Mux
+  io.out := 1.U
   //Implement above ----------
 }