diff --git a/examples/simple/simple.cue b/examples/simple/simple.cue
index 8d8b203d..229e6ce2 100644
--- a/examples/simple/simple.cue
+++ b/examples/simple/simple.cue
@@ -17,7 +17,8 @@ let base=alpine & {
 www: {
 
 	source: {
-		#dagger: compute: []
+		// Make this undefined on purpose to require an input directory.
+		#dagger: compute: _
 	}
 
 	host: string