diff --git a/drools-ansible-rulebook-integration-api/src/test/java/org/drools/ansible/rulebook/integration/api/ProcessEventTest.java b/drools-ansible-rulebook-integration-api/src/test/java/org/drools/ansible/rulebook/integration/api/ProcessEventTest.java
index 770e3bb6..ee6a222c 100644
--- a/drools-ansible-rulebook-integration-api/src/test/java/org/drools/ansible/rulebook/integration/api/ProcessEventTest.java
+++ b/drools-ansible-rulebook-integration-api/src/test/java/org/drools/ansible/rulebook/integration/api/ProcessEventTest.java
@@ -1,5 +1,6 @@
 package org.drools.ansible.rulebook.integration.api;
 
+import org.drools.base.reteoo.InitialFactImpl;
 import org.junit.Test;
 import org.kie.api.prototype.PrototypeFactInstance;
 import org.kie.api.runtime.rule.Match;
@@ -131,4 +132,51 @@ public void testEventShouldProduceMultipleMatchesForSameRule() {
 
         rulesExecutor.dispose();
     }
+
+    public static final String JSON_IS_NOT_DEFINED =
+            """
+            {
+                "rules": [
+                        {
+                            "Rule": {
+                                "name": "r1",
+                                "condition": {
+                                    "AllCondition": [
+                                        {
+                                            "IsNotDefinedExpression": {
+                                                "Event": "beta.xheaders.age"
+                                            }
+                                        }
+                                    ]
+                                },
+                                "actions": [
+                                    {
+                                        "Action": {
+                                            "action": "debug",
+                                            "action_args": {}
+                                        }
+                                    }
+                                ],
+                                "enabled": true
+                            }
+                        }
+                    ]
+            }
+            """;
+
+    @Test
+    public void isNotDefinedExpression() {
+        RulesExecutor rulesExecutor = RulesExecutorFactory.createFromJson(JSON_IS_NOT_DEFINED);
+
+        List<Match> matchedRules = rulesExecutor.processEvents("{\"meta\":{\"headers\":{\"token\":123}}}").join();
+        assertEquals(1, matchedRules.size());
+        assertEquals("r1", matchedRules.get(0).getRule().getName());
+        assertEquals(InitialFactImpl.class, matchedRules.get(0).getObjects().get(0).getClass());
+
+        // "isNotDefined" rule matches only once
+        matchedRules = rulesExecutor.processEvents("{\"beta\":{\"headers\":{\"age\":23}}}").join();
+        assertEquals(0, matchedRules.size());
+
+        rulesExecutor.dispose();
+    }
 }