@@ -44,6 +44,7 @@ Project: jackson-databind
 #6339: Enum aliases from an appended `AnnotationIntrospector` are discarded:
   `JacksonAnnotationIntrospector.findEnumAliases()` overwrites every slot
  (reported, fixed by @Marti2203)
+#6399: `NullSerializer` should not be annotated with `@JacksonStdImpl`
 
 2.22.5 (not yet released)
 
 
@@ -5,7 +5,6 @@
 
 import com.fasterxml.jackson.core.*;
 import com.fasterxml.jackson.databind.*;
-import com.fasterxml.jackson.databind.annotation.JacksonStdImpl;
 import com.fasterxml.jackson.databind.jsonFormatVisitors.JsonFormatVisitorWrapper;
 import com.fasterxml.jackson.databind.jsontype.TypeSerializer;
 
@@ -15,8 +14,11 @@
  * Used as the default "null serializer" (which is used for serializing
  * null object references unless overridden), as well as for some
  * more exotic types (java.lang.Void).
+ *<p>
+ * NOTE: not annotated with {@code @JacksonStdImpl} (since 2.23): it is a
+ * fallback rather than the default serializer for any type, and optimization
+ * modules (Afterburner, Blackbird) would otherwise consider it replaceable.
  */
-@JacksonStdImpl
 @SuppressWarnings("serial")
 public class NullSerializer
     extends StdSerializer<Object>
 
@@ -12,6 +12,7 @@
 import com.fasterxml.jackson.databind.ser.DefaultSerializerProvider;
 import com.fasterxml.jackson.databind.ser.SerializerFactory;
 import com.fasterxml.jackson.databind.testutil.DatabindTestUtil;
+import com.fasterxml.jackson.databind.util.ClassUtil;
 
 import static org.junit.jupiter.api.Assertions.*;
 
@@ -138,4 +139,15 @@ public void testNullSerializerForProperty() throws Exception
     {
         assertEquals("{\"a\":\"foobar\"}", MAPPER.writeValueAsString(new BeanWithNullProps()));
     }
+
+    // [databind#6399]: `NullSerializer` is a fallback, not the default serializer
+    // for any type, so must NOT be marked as standard implementation (optimization
+    // modules like Afterburner and Blackbird consider std impls replaceable)
+    @Test
+    public void testNullSerializerNotMarkedAsStdImpl() throws Exception
+    {
+        assertFalse(ClassUtil.isJacksonStdImpl(
+                com.fasterxml.jackson.databind.ser.std.NullSerializer.class),
+                "NullSerializer must not be @JacksonStdImpl (see databind#6399)");
+    }
 }