diff --git a/src/SpecBaseObject.php b/src/SpecBaseObject.php
index ef93401..0d19661 100644
--- a/src/SpecBaseObject.php
+++ b/src/SpecBaseObject.php
@@ -12,6 +12,7 @@
 use cebe\openapi\json\JsonPointer;
 use cebe\openapi\json\JsonReference;
 use cebe\openapi\spec\Reference;
+use cebe\openapi\spec\Schema;
 use cebe\openapi\spec\Type;
 
 /**
@@ -234,7 +235,15 @@ public function validate(): bool
         }
         $this->_recursingValidate = true;
         $valid = true;
-        foreach ($this->_properties as $v) {
+        $allowedFields = array_keys($this->attributes());
+        if (static::class === Schema::class) {
+            $allowedFields[] = 'additionalProperties';
+        }
+        foreach ($this->_properties as $k => $v) {
+            if ($allowedFields && !in_array($k, $allowedFields, true) && substr($k, 0, 2) !== 'x-') {
+                $valid = false;
+                $this->addError('Invalid field: "' . $k . '"');
+            }
             if ($v instanceof SpecObjectInterface) {
                 if (!$v->validate()) {
                     $valid = false;
@@ -275,7 +284,7 @@ public function getErrors(): array
         if (($pos = $this->getDocumentPosition()) !== null) {
             $errors = [
                 array_map(function ($e) use ($pos) {
-                    return "[{$pos->getPointer()}] $e";
+                    return $pos->getPointer() ? "[{$pos->getPointer()}] $e" : $e;
                 }, $this->_errors)
             ];
         } else {
diff --git a/tests/ReaderTest.php b/tests/ReaderTest.php
index eee9595..59e7566 100644
--- a/tests/ReaderTest.php
+++ b/tests/ReaderTest.php
@@ -44,7 +44,12 @@ public function testReadYamlWithAnchors()
         $openApiFile = __DIR__ . '/spec/data/traits-mixins.yaml';
         $openapi = \cebe\openapi\Reader::readFromYamlFile($openApiFile);
 
-        $this->assertApiContent($openapi);
+        $this->assertApiContent($openapi, [
+            '[/paths/~1foo/put/responses/200] Invalid field: "schema"',
+            '[/paths/~1foo/put/responses/404] Invalid field: "schema"',
+            '[/paths/~1foo/put/responses/428] Invalid field: "schema"',
+            '[/paths/~1foo/put/responses/default] Invalid field: "schema"'
+        ]);
 
         $putOperation = $openapi->paths['/foo']->put;
         $this->assertEquals('create foo', $putOperation->description);
@@ -77,11 +82,11 @@ public function testReadYamlWithAnchors()
         $this->assertEquals('uuid of the resource', $foo->properties['uuid']->description);
     }
 
-    private function assertApiContent(\cebe\openapi\spec\OpenApi $openapi)
+    private function assertApiContent(\cebe\openapi\spec\OpenApi $openapi, $expected = [])
     {
         $result = $openapi->validate();
-        $this->assertEquals([], $openapi->getErrors());
-        $this->assertTrue($result);
+        $this->assertEquals($expected, $openapi->getErrors());
+        $expected ? $this->assertFalse($result) : $this->assertTrue($result);
 
 
         $this->assertEquals("3.0.0", $openapi->openapi);
diff --git a/tests/spec/OpenApiTest.php b/tests/spec/OpenApiTest.php
index 433bd35..f7001e4 100644
--- a/tests/spec/OpenApiTest.php
+++ b/tests/spec/OpenApiTest.php
@@ -233,4 +233,27 @@ public function testSpecs($openApiFile)
         }
 
     }
+
+    public function testInvalidTopLevelField()
+    {
+        $openapi = new OpenApi([
+            'AAAAAcomponents' => [
+                'User' => [
+                    'type' => 'object',
+                    'properties' => [
+                        'id' => [
+                            'type' => 'integer'
+                        ],
+                    ]
+                ]
+            ]
+        ]);
+        $this->assertFalse($openapi->validate());
+        $this->assertEquals([
+            'Invalid field: "AAAAAcomponents"',
+            'OpenApi is missing required property: openapi',
+            'OpenApi is missing required property: info',
+            'OpenApi is missing required property: paths',
+        ], $openapi->getErrors());
+    }
 }
diff --git a/tests/spec/PathTest.php b/tests/spec/PathTest.php
index 2d47e3e..2464fec 100644
--- a/tests/spec/PathTest.php
+++ b/tests/spec/PathTest.php
@@ -141,8 +141,11 @@ public function testPathItemReference()
         $openapi = Reader::readFromYamlFile($file, \cebe\openapi\spec\OpenApi::class, false);
 
         $result = $openapi->validate();
-        $this->assertEquals([], $openapi->getErrors(), print_r($openapi->getErrors(), true));
-        $this->assertTrue($result);
+        $this->assertEquals([
+            'Invalid field: "X-EXTENSION"',
+            'Invalid field: "xyz-extension"'
+        ], $openapi->getErrors(), print_r($openapi->getErrors(), true));
+        $this->assertFalse($result);
 
         $this->assertInstanceOf(Paths::class, $openapi->paths);
         $this->assertInstanceOf(PathItem::class, $fooPath = $openapi->paths['/foo']);
@@ -177,8 +180,11 @@ public function testPathItemReference()
         $openapi = Reader::readFromYamlFile($file, \cebe\openapi\spec\OpenApi::class, true);
 
         $result = $openapi->validate();
-        $this->assertEquals([], $openapi->getErrors(), print_r($openapi->getErrors(), true));
-        $this->assertTrue($result);
+        $this->assertEquals([
+            'Invalid field: "X-EXTENSION"',
+            'Invalid field: "xyz-extension"'
+        ], $openapi->getErrors());
+        $this->assertFalse($result);
 
         $this->assertInstanceOf(Paths::class, $openapi->paths);
         $this->assertInstanceOf(PathItem::class, $fooPath = $openapi->paths['/foo']);
diff --git a/tests/spec/SchemaTest.php b/tests/spec/SchemaTest.php
index 1600b3b..684c1d7 100644
--- a/tests/spec/SchemaTest.php
+++ b/tests/spec/SchemaTest.php
@@ -172,6 +172,7 @@ public function testDiscriminator()
 
         $result = $schema->validate();
         $this->assertEquals([
+            'Invalid field: "map"',
             'Discriminator is missing required property: propertyName'
         ], $schema->getErrors());
         $this->assertFalse($result);