Don't run checks which assume a defined instance against undefined

src/JsonSchema/Constraints/UndefinedConstraint.php

@@ -153,6 +153,12 @@ protected function validateCommonProperties(&$value, $schema = null, JsonPointer
                     $propertyName = end($propertyPaths);
                     $this->addError(ConstraintError::REQUIRED(), $path, array('property' => $propertyName));
                 }
+            } else {
+                // if the value is both undefined and not required, skip remaining checks
+                // in this method which assume an actual, defined instance when validating.
+                if ($value instanceof self) {
+                    return;
+                }
             }
         }
 

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"
+                            }
+                        }
+                    }
+                }'
             )
         );
     }