diff --git a/src/Optimizely/ProjectConfig.php b/src/Optimizely/ProjectConfig.php
index fd172299..4ae69616 100644
--- a/src/Optimizely/ProjectConfig.php
+++ b/src/Optimizely/ProjectConfig.php
@@ -1,6 +1,6 @@
 <?php
 /**
- * Copyright 2016, 2018, Optimizely
+ * Copyright 2016, 2018-2019 Optimizely
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -455,7 +455,7 @@ public function getEvent($eventKey)
      * @param $audienceId string ID of the audience.
      *
      * @return Audience Entity corresponding to the ID.
-     *         Dummy entity is returned if ID is invalid.
+     *         Null is returned if ID is invalid.
      */
     public function getAudience($audienceId)
     {
@@ -465,7 +465,8 @@ public function getAudience($audienceId)
 
         $this->_logger->log(Logger::ERROR, sprintf('Audience ID "%s" is not in datafile.', $audienceId));
         $this->_errorHandler->handleError(new InvalidAudienceException('Provided audience is not in datafile.'));
-        return new Audience();
+
+        return null;
     }
 
     /**
diff --git a/src/Optimizely/Utils/Validator.php b/src/Optimizely/Utils/Validator.php
index 8800a07c..dd6f8eeb 100644
--- a/src/Optimizely/Utils/Validator.php
+++ b/src/Optimizely/Utils/Validator.php
@@ -1,6 +1,6 @@
 <?php
 /**
- * Copyright 2016-2018, Optimizely
+ * Copyright 2016-2019, Optimizely
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -160,7 +160,12 @@ public static function isUserInExperiment($config, $experiment, $userAttributes)
 
         $evaluateAudience = function($audienceId) use ($config, $evaluateCustomAttr) {
             $conditionTreeEvaluator = new ConditionTreeEvaluator();
+
             $audience = $config->getAudience($audienceId);
+            if ($audience === null) {
+                return null;
+            }
+
             return $conditionTreeEvaluator->evaluate($audience->getConditionsList(), $evaluateCustomAttr);
         };
 
diff --git a/tests/ProjectConfigTest.php b/tests/ProjectConfigTest.php
index b19c7526..82de415f 100644
--- a/tests/ProjectConfigTest.php
+++ b/tests/ProjectConfigTest.php
@@ -1,6 +1,6 @@
 <?php
 /**
- * Copyright 2016-2018, Optimizely
+ * Copyright 2016-2019, Optimizely
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -546,7 +546,7 @@ public function testGetAudienceInvalidKey()
             ->method('handleError')
             ->with(new InvalidAudienceException('Provided audience is not in datafile.'));
 
-        $this->assertEquals(new Audience(), $this->config->getAudience('invalid_id'));
+        $this->assertNull($this->config->getAudience('invalid_id'));
     }
 
     public function testGetAttributeValidKey()
diff --git a/tests/UtilsTests/ValidatorTest.php b/tests/UtilsTests/ValidatorTest.php
index b0909a14..be1e9769 100644
--- a/tests/UtilsTests/ValidatorTest.php
+++ b/tests/UtilsTests/ValidatorTest.php
@@ -1,6 +1,6 @@
 <?php
 /**
- * Copyright 2016-2018, Optimizely
+ * Copyright 2016-2019, Optimizely
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -346,6 +346,25 @@ public function testIsUserInExperimentWithAudienceConditionsSetToAudienceIdStrin
         );
     }
 
+    public function testIsUserInExperimentWithUnknownAudienceId()
+    {
+        $config = new ProjectConfig(DATAFILE, $this->loggerMock, new NoOpErrorHandler());
+        $experiment = $config->getExperimentFromKey('test_experiment');
+
+        // Both audience Ids and audience conditions exist. Audience Ids is ignored.
+        $experiment->setAudienceIds([]);
+        $experiment->setAudienceConditions(["or", "unknown_audience_id", "7718080042"]);
+        
+        // User qualifies for audience with ID "7718080042".
+        $this->assertTrue(
+            Validator::isUserInExperiment(
+                $config,
+                $experiment,
+                ['device_type' => 'iPhone', 'location' => 'San Francisco']
+            )
+        );
+    }
+
     // test that isUserInExperiment evaluates simple audience.
     public function testIsUserInExperimentWithSimpleAudience()
     {