diff --git a/response_writer.go b/response_writer.go
index 9035e6f1fd..8e2d8b3099 100644
--- a/response_writer.go
+++ b/response_writer.go
@@ -114,15 +114,22 @@ func (w *responseWriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
 	if w.size > 0 {
 		return nil, nil, errHijackAlreadyWritten
 	}
+	hijacker, ok := w.ResponseWriter.(http.Hijacker)
+	if !ok {
+		return nil, nil, http.ErrNotSupported
+	}
 	if w.size < 0 {
 		w.size = 0
 	}
-	return w.ResponseWriter.(http.Hijacker).Hijack()
+	return hijacker.Hijack()
 }
 
 // CloseNotify implements the http.CloseNotifier interface.
 func (w *responseWriter) CloseNotify() <-chan bool {
-	return w.ResponseWriter.(http.CloseNotifier).CloseNotify()
+	if cn, ok := w.ResponseWriter.(http.CloseNotifier); ok {
+		return cn.CloseNotify()
+	}
+	return nil
 }
 
 // Flush implements the http.Flusher interface.
diff --git a/response_writer_test.go b/response_writer_test.go
index dfc1d2c60d..83d3fc8b06 100644
--- a/response_writer_test.go
+++ b/response_writer_test.go
@@ -113,15 +113,18 @@ func TestResponseWriterHijack(t *testing.T) {
 	writer.reset(testWriter)
 	w := ResponseWriter(writer)
 
-	assert.Panics(t, func() {
-		_, _, err := w.Hijack()
-		require.NoError(t, err)
-	})
-	assert.True(t, w.Written())
+	// httptest.ResponseRecorder doesn't implement http.Hijacker; return
+	// http.ErrNotSupported instead of panicking (#4638). On unsupported the
+	// writer state stays untouched so the handler can still emit a normal
+	// HTTP response as a fallback.
+	conn, buf, err := w.Hijack()
+	assert.Nil(t, conn)
+	assert.Nil(t, buf)
+	require.ErrorIs(t, err, http.ErrNotSupported)
+	assert.False(t, w.Written())
 
-	assert.Panics(t, func() {
-		w.CloseNotify()
-	})
+	// CloseNotify on a non-CloseNotifier returns nil instead of panicking.
+	assert.Nil(t, w.CloseNotify())
 
 	w.Flush()
 }