diff --git a/vaadin-radio-button-flow-parent/vaadin-radio-button-flow/src/main/java/com/vaadin/flow/component/radiobutton/RadioButtonGroup.java b/vaadin-radio-button-flow-parent/vaadin-radio-button-flow/src/main/java/com/vaadin/flow/component/radiobutton/RadioButtonGroup.java
index dccc459c0b4..23e815bd376 100755
--- a/vaadin-radio-button-flow-parent/vaadin-radio-button-flow/src/main/java/com/vaadin/flow/component/radiobutton/RadioButtonGroup.java
+++ b/vaadin-radio-button-flow-parent/vaadin-radio-button-flow/src/main/java/com/vaadin/flow/component/radiobutton/RadioButtonGroup.java
@@ -608,9 +608,15 @@ private void rebuild() {
             // Ignore new size requests unless the last one has been executed
             // so as to avoid multiple beforeClientResponses.
             if (sizeRequest == null) {
-                sizeRequest = ui -> {
-                    fireSizeEvent();
-                    sizeRequest = null;
+                // Using anonymous class to fix serialization issue:
+                // https://github.com/vaadin/flow-components/issues/6555
+                // Do not replace with lambda
+                sizeRequest = new SerializableConsumer<>() {
+                    @Override
+                    public void accept(UI ui) {
+                        fireSizeEvent();
+                        sizeRequest = null;
+                    }
                 };
                 // Size event is fired before client response so as to avoid
                 // multiple size change events during server round trips
diff --git a/vaadin-radio-button-flow-parent/vaadin-radio-button-flow/src/test/java/com/vaadin/flow/component/radiobutton/tests/RadioButtonSerializableTest.java b/vaadin-radio-button-flow-parent/vaadin-radio-button-flow/src/test/java/com/vaadin/flow/component/radiobutton/tests/RadioButtonSerializableTest.java
index 1ad541252f2..ca5d3c1c0de 100644
--- a/vaadin-radio-button-flow-parent/vaadin-radio-button-flow/src/test/java/com/vaadin/flow/component/radiobutton/tests/RadioButtonSerializableTest.java
+++ b/vaadin-radio-button-flow-parent/vaadin-radio-button-flow/src/test/java/com/vaadin/flow/component/radiobutton/tests/RadioButtonSerializableTest.java
@@ -15,7 +15,23 @@
  */
 package com.vaadin.flow.component.radiobutton.tests;
 
+import org.junit.Test;
+
+import com.vaadin.flow.component.UI;
+import com.vaadin.flow.component.radiobutton.RadioButtonGroup;
 import com.vaadin.flow.testutil.ClassesSerializableTest;
 
 public class RadioButtonSerializableTest extends ClassesSerializableTest {
+    @Test
+    public void setItems_addToUI_radioButtonGroupIsSerializable()
+            throws Throwable {
+        var group = new RadioButtonGroup<>();
+        group.setItems("Item 1", "Item 2");
+
+        var ui = new UI();
+        UI.setCurrent(ui);
+        ui.add(group);
+
+        serializeAndDeserialize(ui);
+    }
 }