diff --git a/.php_cs.dist b/.php_cs.dist
index 0b5e2b2c..5a7a578e 100644
--- a/.php_cs.dist
+++ b/.php_cs.dist
@@ -21,8 +21,11 @@ $config
         'phpdoc_order' => true,
         'phpdoc_summary' => false,
         'pre_increment' => false,
+        'increment_style' => false,
         'simplified_null_return' => false,
         'trailing_comma_in_multiline_array' => false,
+        'yoda_style' => false,
+        'phpdoc_types_order' => array('null_adjustment' => 'none', 'sort_algorithm' => 'none'),
     ))
     ->setFinder($finder)
 ;
diff --git a/.travis.yml b/.travis.yml
index 3b4f5fc1..be79e9fe 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -12,12 +12,17 @@ matrix:
     - php: 5.3
       dist: precise
     - php: 5.4
+      dist: trusty
     - php: 5.5
+      dist: trusty
     - php: 5.6
     - php: 7.0
-      env: WITH_COVERAGE=true WITH_PHPCSFIXER=true
+      env: WITH_COVERAGE=true
+    - php: 7.0
+      env: WITH_PHPCSFIXER=true
     - php: 7.1
     - php: 7.2
+    - php: 7.3
     - php: 'nightly'
     - php: hhvm
       dist: trusty
diff --git a/composer.json b/composer.json
index 20e85fc3..884a6971 100644
--- a/composer.json
+++ b/composer.json
@@ -30,7 +30,7 @@
         "php": ">=5.3.3"
     },
     "require-dev": {
-        "friendsofphp/php-cs-fixer": "~2.2.20",
+        "friendsofphp/php-cs-fixer": "~2.2.20||~2.15.1",
         "json-schema/JSON-Schema-Test-Suite": "1.2.0",
         "phpunit/phpunit": "^4.8.35"
     },
diff --git a/src/JsonSchema/Constraints/UndefinedConstraint.php b/src/JsonSchema/Constraints/UndefinedConstraint.php
index 257db155..e194cb0e 100644
--- a/src/JsonSchema/Constraints/UndefinedConstraint.php
+++ b/src/JsonSchema/Constraints/UndefinedConstraint.php
@@ -150,6 +150,12 @@ protected function validateCommonProperties(&$value, $schema = null, JsonPointer
                         'required'
                     );
                 }
+            } else {
+                // If the value is both undefined and not required, skip remaining checks
+                // in this method which assume an actual, defined instance when validating.
+                if ($value instanceof self) {
+                    return;
+                }
             }
         }
 
diff --git a/src/JsonSchema/Validator.php b/src/JsonSchema/Validator.php
index 448091cd..431b3781 100644
--- a/src/JsonSchema/Validator.php
+++ b/src/JsonSchema/Validator.php
@@ -34,9 +34,7 @@ class Validator extends BaseConstraint
      * Both the php object and the schema are supposed to be a result of a json_decode call.
      * The validation works as defined by the schema proposal in http://json-schema.org.
      *
-     * Note that the first argument is passwd by reference, so you must pass in a variable.
-     *
-     * {@inheritdoc}
+     * Note that the first argument is passed by reference, so you must pass in a variable.
      */
     public function validate(&$value, $schema = null, $checkMode = null)
     {
diff --git a/tests/Constraints/ArraysTest.php b/tests/Constraints/ArraysTest.php
index dac14358..63b97bf1 100644
--- a/tests/Constraints/ArraysTest.php
+++ b/tests/Constraints/ArraysTest.php
@@ -71,6 +71,51 @@ public function getInvalidTests()
                         }
                     }
                 }'
+            ),
+            array( // Test array items.enum where type string fail validation if value(s) is/are not in items.enum
+                '{"data": ["a", "b"]}',
+                '{
+                    "type": "object",
+                    "properties": {
+                        "data": {
+                            "type": "array",
+                            "items": {
+                                "type": "string",
+                                "enum": ["b", "c"]
+                            }
+                        }
+                    }
+                }'
+            ),
+            array( // Test array items.enum where type integer fail validation if value(s) is/are not in items.enum
+                '{"data": [1, 2]}',
+                '{
+                    "type": "object",
+                    "properties": {
+                        "data": {
+                            "type": "array",
+                            "items": {
+                                "type": "integer",
+                                "enum": [2, 3]
+                            }
+                        }
+                    }
+                }'
+            ),
+            array( // Test array items.enum where type number fail validation if value(s) is/are not in items.enum
+                '{"data": [1.25, 2.25]}',
+                '{
+                    "type": "object",
+                    "properties": {
+                        "data": {
+                            "type": "array",
+                            "items": {
+                                "type": "number",
+                                "enum": [1.25, 2]
+                            }
+                        }
+                    }
+                }'
             )
         );
     }
@@ -168,6 +213,51 @@ public function getValidTests()
                         }
                     }
                 }'
+            ),
+            array( // Test array items.enum where type string passes validation if value(s) is/are in items.enum
+                '{"data": ["c", "c", "b"]}',
+                '{
+                    "type": "object",
+                    "properties": {
+                        "data": {
+                            "type": "array",
+                            "items": {
+                                "type": "string",
+                                "enum": ["b", "c"]
+                            }
+                        }
+                    }
+                }'
+            ),
+            array( // Test array items.enum where type integer passes validation if value(s) is/are in items.enum
+                '{"data": [1, 1, 2]}',
+                '{
+                    "type": "object",
+                    "properties": {
+                        "data": {
+                            "type": "array",
+                            "items": {
+                                "type": "integer",
+                                "enum": [1, 2]
+                            }
+                        }
+                    }
+                }'
+            ),
+            array( // Test array items.enum where type number passes validation if value(s) is/are in items.enum
+                '{"data": [1.25, 1.25, 2.25]}',
+                '{
+                    "type": "object",
+                    "properties": {
+                        "data": {
+                            "type": "array",
+                            "items": {
+                                "type": "number",
+                                "enum": [1.25, 2.25]
+                            }
+                        }
+                    }
+                }'
             )
         );
     }
diff --git a/tests/Constraints/NotTest.php b/tests/Constraints/NotTest.php
index 3a950f57..6183f684 100644
--- a/tests/Constraints/NotTest.php
+++ b/tests/Constraints/NotTest.php
@@ -31,6 +31,20 @@ public function getInvalidTests()
                         }
                     }
                 }'
+            ),
+            array( // check that a missing, required property is correctly validated
+                '{"y": "foo"}',
+                '{
+                    "type": "object",
+                    "required": ["x"],
+                    "properties": {
+                        "x": {
+                            "not": {
+                                "type": "null"
+                            }
+                        }
+                    }
+                }'
             )
         );
     }
@@ -69,6 +83,19 @@ public function getValidTests()
                         }
                     }
                 }'
+            ),
+            array( // check that a missing, non-required property isn't validated
+                '{"y": "foo"}',
+                '{
+                    "type": "object",
+                    "properties": {
+                        "x": {
+                            "not": {
+                                "type": "null"
+                            }
+                        }
+                    }
+                }'
             )
         );
     }