diff --git a/container_option.go b/container_option.go
index f6013e7..bf901a9 100644
--- a/container_option.go
+++ b/container_option.go
@@ -84,6 +84,9 @@
 
 // WithDebugOutput sets debug output.
 func WithDebugOutput(w io.Writer) ContainerOption {
+	if w == nil {
+		w = io.Discard
+	}
 	return func(s *pState) {
 		s.debugOut = w
 	}
diff --git a/progress.go b/progress.go
index 4dc6129..6d1c6af 100644
--- a/progress.go
+++ b/progress.go
@@ -81,6 +81,7 @@
 		queueBars:   make(map[*Bar]*Bar),
 		output:      os.Stdout,
 		popPriority: math.MinInt32,
+		debugOut:    io.Discard,
 	}
 
 	for _, opt := range options {