diff --git a/lesson-templates/lesson-04-functions.tm b/lesson-templates/lesson-04-functions.tm
index 83d3b9e..f74a750 100644
--- a/lesson-templates/lesson-04-functions.tm
+++ b/lesson-templates/lesson-04-functions.tm
@@ -23,4 +23,4 @@ func main():
 func add(x:Int, y:Int -> Int):
 
     # Fix this so it returns a sensible result:
-    return 0
+    return ???