diff --git a/src/main/java/com/ankurm/jackson3/part5polymorphic/F04UnknownTypeId.java b/src/main/java/com/ankurm/jackson3/part5polymorphic/F04UnknownTypeId.java
new file mode 100644
index 0000000..f37d515
--- /dev/null
+++ b/src/main/java/com/ankurm/jackson3/part5polymorphic/F04UnknownTypeId.java
@@ -0,0 +1,36 @@
+package com.ankurm.jackson3.part5polymorphic;
+
+import tools.jackson.databind.json.JsonMapper;
+
+/**
+ * BEYOND THE POST — what actually happens when the discriminator is wrong.
+ *
+ * The security post asserts that an unregistered type name throws
+ * InvalidTypeIdException. This runs the three failure modes so the exception types
+ * are on the record rather than assumed: unknown name, attacker-supplied class name,
+ * and a missing discriminator.
+ */
+public class F04UnknownTypeId {
+
+    public static void main(String[] args) {
+        JsonMapper mapper = JsonMapper.builder().build();
+
+        attempt(mapper, "unknown logical name",
+                "{\"paymentType\":\"crypto\",\"paymentId\":9,\"amountDue\":1.0}");
+
+        attempt(mapper, "attacker-supplied class name",
+                "{\"paymentType\":\"com.malicious.Gadget\",\"paymentId\":9,\"amountDue\":1.0}");
+
+        attempt(mapper, "missing discriminator",
+                "{\"paymentId\":9,\"amountDue\":1.0}");
+    }
+
+    private static void attempt(JsonMapper mapper, String label, String json) {
+        try {
+            PaymentMethod result = mapper.readValue(json, PaymentMethod.class);
+            System.out.printf("%-30s -> UNEXPECTEDLY OK: %s%n", label, result);
+        } catch (Exception e) {
+            System.out.printf("%-30s -> %s%n", label, e.getClass().getSimpleName());
+        }
+    }
+}