diff --git a/api.go b/api.go
index b45f1a35d..127ad7760 100644
--- a/api.go
+++ b/api.go
@@ -169,9 +169,7 @@ func getContainersExport(srv *Server, version float64, w http.ResponseWriter, r
 		return fmt.Errorf("Missing parameter")
 	}
 	job := srv.Eng.Job("export", vars["name"])
-	if err := job.Stdout.Add(w); err != nil {
-		return err
-	}
+	job.Stdout.Add(w)
 	if err := job.Run(); err != nil {
 		return err
 	}
@@ -573,9 +571,7 @@ func getImagesGet(srv *Server, version float64, w http.ResponseWriter, r *http.R
 		w.Header().Set("Content-Type", "application/x-tar")
 	}
 	job := srv.Eng.Job("image_export", vars["name"])
-	if err := job.Stdout.Add(w); err != nil {
-		return err
-	}
+	job.Stdout.Add(w)
 	return job.Run()
 }
 
@@ -806,7 +802,7 @@ func postContainersAttach(srv *Server, version float64, w http.ResponseWriter, r
 	job.Setenv("stderr", r.Form.Get("stderr"))
 	job.Stdin.Add(inStream)
 	job.Stdout.Add(outStream)
-	job.Stderr.Add(errStream)
+	job.Stderr.Set(errStream)
 	if err := job.Run(); err != nil {
 		fmt.Fprintf(outStream, "Error: %s\n", err)
 
@@ -836,7 +832,7 @@ func wsContainersAttach(srv *Server, version float64, w http.ResponseWriter, r *
 		job.Setenv("stderr", r.Form.Get("stderr"))
 		job.Stdin.Add(ws)
 		job.Stdout.Add(ws)
-		job.Stderr.Add(ws)
+		job.Stderr.Set(ws)
 		if err := job.Run(); err != nil {
 			utils.Errorf("Error: %s", err)
 		}
diff --git a/engine/streams.go b/engine/streams.go
index 703a56256..4b1f172b4 100644
--- a/engine/streams.go
+++ b/engine/streams.go
@@ -23,20 +23,28 @@ func NewOutput() *Output {
 
 // Return true if something was written on this output
 func (o *Output) Used() bool {
-	o.Mutex.Lock()
-	defer o.Mutex.Unlock()
+	o.Lock()
+	defer o.Unlock()
 	return o.used
 }
 
 // Add attaches a new destination to the Output. Any data subsequently written
 // to the output will be written to the new destination in addition to all the others.
 // This method is thread-safe.
-// FIXME: Add cannot fail
-func (o *Output) Add(dst io.Writer) error {
-	o.Mutex.Lock()
-	defer o.Mutex.Unlock()
+func (o *Output) Add(dst io.Writer) {
+	o.Lock()
+	defer o.Unlock()
 	o.dests = append(o.dests, dst)
-	return nil
+}
+
+// Set closes and remove existing destination and then attaches a new destination to
+// the Output. Any data subsequently written to the output will be written to the new
+// destination in addition to all the others. This method is thread-safe.
+func (o *Output) Set(dst io.Writer) {
+	o.Close()
+	o.Lock()
+	defer o.Unlock()
+	o.dests = []io.Writer{dst}
 }
 
 // AddPipe creates an in-memory pipe with io.Pipe(), adds its writing end as a destination,
@@ -88,8 +96,8 @@ func (o *Output) AddString(dst *string) error {
 // Write writes the same data to all registered destinations.
 // This method is thread-safe.
 func (o *Output) Write(p []byte) (n int, err error) {
-	o.Mutex.Lock()
-	defer o.Mutex.Unlock()
+	o.Lock()
+	defer o.Unlock()
 	o.used = true
 	var firstErr error
 	for _, dst := range o.dests {
@@ -105,8 +113,8 @@ func (o *Output) Write(p []byte) (n int, err error) {
 // AddTail and AddString tasks to complete.
 // The Close method of each destination is called if it exists.
 func (o *Output) Close() error {
-	o.Mutex.Lock()
-	defer o.Mutex.Unlock()
+	o.Lock()
+	defer o.Unlock()
 	var firstErr error
 	for _, dst := range o.dests {
 		if closer, ok := dst.(io.WriteCloser); ok {
diff --git a/engine/streams_test.go b/engine/streams_test.go
index 37720c61e..30d31d295 100644
--- a/engine/streams_test.go
+++ b/engine/streams_test.go
@@ -95,9 +95,7 @@ func TestOutputAddEnv(t *testing.T) {
 func TestOutputAddClose(t *testing.T) {
 	o := NewOutput()
 	var s sentinelWriteCloser
-	if err := o.Add(&s); err != nil {
-		t.Fatal(err)
-	}
+	o.Add(&s)
 	if err := o.Close(); err != nil {
 		t.Fatal(err)
 	}