diff --git a/prometheus/promhttp/delegator.go b/prometheus/promhttp/delegator.go
index 659c717..9c1c66d 100644
--- a/prometheus/promhttp/delegator.go
+++ b/prometheus/promhttp/delegator.go
@@ -102,10 +102,10 @@ func init() {
 		return d
 	}
 	pickDelegator[closeNotifier] = func(d *responseWriterDelegator) delegator { // 1
-		return closeNotifierDelegator{d}
+		return &closeNotifierDelegator{d}
 	}
 	pickDelegator[flusher] = func(d *responseWriterDelegator) delegator { // 2
-		return flusherDelegator{d}
+		return &flusherDelegator{d}
 	}
 	pickDelegator[flusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 3
 		return struct {
diff --git a/prometheus/promhttp/delegator_1_8.go b/prometheus/promhttp/delegator_1_8.go
index f4d386f..75a905e 100644
--- a/prometheus/promhttp/delegator_1_8.go
+++ b/prometheus/promhttp/delegator_1_8.go
@@ -28,7 +28,7 @@ func (d *pusherDelegator) Push(target string, opts *http.PushOptions) error {
 
 func init() {
 	pickDelegator[pusher] = func(d *responseWriterDelegator) delegator { // 16
-		return pusherDelegator{d}
+		return &pusherDelegator{d}
 	}
 	pickDelegator[pusher+closeNotifier] = func(d *responseWriterDelegator) delegator { // 17
 		return struct {
diff --git a/prometheus/promhttp/instrument_server_test.go b/prometheus/promhttp/instrument_server_test.go
index e9af63e..716c6f4 100644
--- a/prometheus/promhttp/instrument_server_test.go
+++ b/prometheus/promhttp/instrument_server_test.go
@@ -281,6 +281,16 @@ func (t *testResponseWriter) ReadFrom(io.Reader) (int64, error) {
 	return 0, nil
 }
 
+// testFlusher is an http.ResponseWriter that also implements http.Flusher.
+type testFlusher struct {
+	flushCalled bool
+}
+
+func (t *testFlusher) Header() http.Header       { return nil }
+func (t *testFlusher) Write([]byte) (int, error) { return 0, nil }
+func (t *testFlusher) WriteHeader(int)           {}
+func (t *testFlusher) Flush()                    { t.flushCalled = true }
+
 func TestInterfaceUpgrade(t *testing.T) {
 	w := &testResponseWriter{}
 	d := newDelegator(w, nil)
@@ -299,6 +309,22 @@ func TestInterfaceUpgrade(t *testing.T) {
 	if _, ok := d.(http.Hijacker); ok {
 		t.Error("delegator unexpectedly implements http.Hijacker")
 	}
+
+	f := &testFlusher{}
+	d = newDelegator(f, nil)
+	if _, ok := d.(http.CloseNotifier); ok {
+		t.Error("delegator unexpectedly implements http.CloseNotifier")
+	}
+	d.(http.Flusher).Flush()
+	if !w.flushCalled {
+		t.Error("Flush not called")
+	}
+	if _, ok := d.(io.ReaderFrom); ok {
+		t.Error("delegator unexpectedly implements io.ReaderFrom")
+	}
+	if _, ok := d.(http.Hijacker); ok {
+		t.Error("delegator unexpectedly implements http.Hijacker")
+	}
 }
 
 func ExampleInstrumentHandlerDuration() {