diff --git a/core/lib/Drupal/Core/PageCache/ChainRequestPolicy.php b/core/lib/Drupal/Core/PageCache/ChainRequestPolicy.php
index c88e1ea..ea3b818 100644
--- a/core/lib/Drupal/Core/PageCache/ChainRequestPolicy.php
+++ b/core/lib/Drupal/Core/PageCache/ChainRequestPolicy.php
@@ -36,26 +36,23 @@ class ChainRequestPolicy implements ChainRequestPolicyInterface {
    * {@inheritdoc}
    */
   public function check(Request $request) {
-    $deny = FALSE;
-    $allow = FALSE;
+    $overall_result = NULL;
 
     foreach ($this->rules as $rule) {
       $result = $rule->check($request);
       if ($result === static::ALLOW) {
-        $allow = $result;
+        $overall_result = $result;
       }
       elseif ($result === static::DENY) {
-        $deny = $result;
+        $overall_result = $result;
         break;
       }
+      elseif (isset($result)) {
+        throw new \UnexpectedValueException('Return value of RequestPolicyInterface::check() must be one of RequestPolicyInterface::ALLOW, RequestPolicyInterface::DENY or NULL');
+      }
     }
 
-    if ($deny) {
-      return $deny;
-    }
-    elseif ($allow) {
-      return $allow;
-    }
+    return $overall_result;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/PageCache/ChainResponsePolicy.php b/core/lib/Drupal/Core/PageCache/ChainResponsePolicy.php
index c9a334e..20d5e39 100644
--- a/core/lib/Drupal/Core/PageCache/ChainResponsePolicy.php
+++ b/core/lib/Drupal/Core/PageCache/ChainResponsePolicy.php
@@ -35,8 +35,12 @@ class ChainResponsePolicy implements ChainResponsePolicyInterface {
    */
   public function check(Response $response, Request $request) {
     foreach ($this->rules as $rule) {
-      if ($rule->check($response, $request) === static::DENY) {
-        return static::DENY;
+      $result = $rule->check($response, $request);
+      if ($result === static::DENY) {
+        return $result;
+      }
+      elseif (isset($result)) {
+        throw new \UnexpectedValueException('Return value of ResponsePolicyInterface::check() must be one of ResponsePolicyInterface::DENY or NULL');
       }
     }
   }
diff --git a/core/tests/Drupal/Tests/Core/PageCache/ChainRequestPolicyTest.php b/core/tests/Drupal/Tests/Core/PageCache/ChainRequestPolicyTest.php
index fd41185..e3e779c 100644
--- a/core/tests/Drupal/Tests/Core/PageCache/ChainRequestPolicyTest.php
+++ b/core/tests/Drupal/Tests/Core/PageCache/ChainRequestPolicyTest.php
@@ -48,12 +48,31 @@ public function testEmptyChain() {
   }
 
   /**
-   * Asserts that check() returns NULL if none of the rules returns an actionable result.
+   * Asserts that check() returns NULL if a rule returns NULL.
    *
-   * @dataProvider providerChainNoResultOnInvalidReturnValue
    * @covers ::check
    */
-  public function testChainNoResultOnInvalidReturnValue($return_value) {
+  public function testNullRuleChain() {
+    $rule = $this->getMock('Drupal\Core\PageCache\RequestPolicyInterface');
+    $rule->expects($this->once())
+      ->method('check')
+      ->with($this->request)
+      ->will($this->returnValue(NULL));
+
+    $this->policy->addPolicy($rule);
+
+    $result = $this->policy->check($this->request);
+    $this->assertSame(NULL, $result);
+  }
+
+  /**
+   * Asserts that check() throws an exception if a rule returns an invalid value.
+   *
+   * @expectedException \UnexpectedValueException
+   * @dataProvider providerChainExceptionOnInvalidReturnValue
+   * @covers ::check
+   */
+  public function testChainExceptionOnInvalidReturnValue($return_value) {
     $rule = $this->getMock('Drupal\Core\PageCache\RequestPolicyInterface');
     $rule->expects($this->once())
       ->method('check')
@@ -62,19 +81,17 @@ public function testChainNoResultOnInvalidReturnValue($return_value) {
 
     $this->policy->addPolicy($rule);
 
-    $actual_result = $this->policy->check($this->request);
-    $this->assertSame(NULL, $actual_result);
+    $this->policy->check($this->request);
   }
 
   /**
-   * Provides test data for testChainNoResultOnInvalidReturnValue.
+   * Provides test data for testChainExceptionOnInvalidReturnValue.
    *
    * @return array
    *   Test input and expected result.
    */
-  public function providerChainNoResultOnInvalidReturnValue() {
+  public function providerChainExceptionOnInvalidReturnValue() {
     return [
-      [NULL],
       [FALSE],
       [0],
       [1],
@@ -115,12 +132,6 @@ public function providerAllowIfAnyRuleReturnedAllow() {
     return [
       [[RequestPolicyInterface::ALLOW]],
       [[NULL, RequestPolicyInterface::ALLOW]],
-      [[RequestPolicyInterface::ALLOW, FALSE]],
-      [[0, RequestPolicyInterface::ALLOW]],
-      [[RequestPolicyInterface::ALLOW, 1]],
-      [[TRUE, RequestPolicyInterface::ALLOW]],
-      [[RequestPolicyInterface::ALLOW, [1, 2, 3]]],
-      [[new \stdClass(), RequestPolicyInterface::ALLOW]],
     ];
   }
 
diff --git a/core/tests/Drupal/Tests/Core/PageCache/ChainResponsePolicyTest.php b/core/tests/Drupal/Tests/Core/PageCache/ChainResponsePolicyTest.php
index 8195e4c..2dbfba3 100644
--- a/core/tests/Drupal/Tests/Core/PageCache/ChainResponsePolicyTest.php
+++ b/core/tests/Drupal/Tests/Core/PageCache/ChainResponsePolicyTest.php
@@ -57,12 +57,31 @@ public function testEmptyChain() {
   }
 
   /**
-   * Asserts that check() returns NULL if none of the rules returns an actionable result.
+   * Asserts that check() returns NULL if a rule returns NULL.
    *
-   * @dataProvider providerChainNoResultOnInvalidReturnValue
    * @covers ::check
    */
-  public function testChainNoResultOnInvalidReturnValue($return_value) {
+  public function testNullRuleChain() {
+    $rule = $this->getMock('Drupal\Core\PageCache\ResponsePolicyInterface');
+    $rule->expects($this->once())
+      ->method('check')
+      ->with($this->response, $this->request)
+      ->will($this->returnValue(NULL));
+
+    $this->policy->addPolicy($rule);
+
+    $result = $this->policy->check($this->response, $this->request);
+    $this->assertSame(NULL, $result);
+  }
+
+  /**
+   * Asserts that check() throws an exception if a rule returns an invalid value.
+   *
+   * @expectedException \UnexpectedValueException
+   * @dataProvider providerChainExceptionOnInvalidReturnValue
+   * @covers ::check
+   */
+  public function testChainExceptionOnInvalidReturnValue($return_value) {
     $rule = $this->getMock('Drupal\Core\PageCache\ResponsePolicyInterface');
     $rule->expects($this->once())
       ->method('check')
@@ -76,14 +95,13 @@ public function testChainNoResultOnInvalidReturnValue($return_value) {
   }
 
   /**
-   * Provides test data for testChainNoResultOnInvalidReturnValue.
+   * Provides test data for testChainExceptionOnInvalidReturnValue.
    *
    * @return array
    *   Test input and expected result.
    */
-  public function providerChainNoResultOnInvalidReturnValue() {
+  public function providerChainExceptionOnInvalidReturnValue() {
     return [
-      [NULL],
       [FALSE],
       [0],
       [1],