diff --git a/README.md b/README.md
index a971d46b..03eec96d 100644
--- a/README.md
+++ b/README.md
@@ -355,7 +355,7 @@ $rules[] = Rule::allClasses()
 ```php
 $rules[] = Rule::allClasses()
     ->that(new ResideInOneOfTheseNamespaces('App\Domain'))
-    ->should(new NotHaveDependencyOutsideNamespace('App\Domain', ['Ramsey\Uuid']))
+    ->should(new NotHaveDependencyOutsideNamespace('App\Domain', ['Ramsey\Uuid'], true))
     ->because('we want protect our domain except for Ramsey\Uuid');
 ```
 
diff --git a/src/Analyzer/FileParser.php b/src/Analyzer/FileParser.php
index d343fd97..7684f306 100644
--- a/src/Analyzer/FileParser.php
+++ b/src/Analyzer/FileParser.php
@@ -35,7 +35,7 @@ public function __construct(
 
         $lexer = new Emulative([
             'usedAttributes' => ['comments', 'startLine', 'endLine', 'startTokenPos', 'endTokenPos'],
-             'phpVersion' => $targetPhpVersion->get() ?? phpversion(),
+            'phpVersion' => $targetPhpVersion->get() ?? phpversion(),
         ]);
 
         $this->parser = (new ParserFactory())->create(ParserFactory::PREFER_PHP7, $lexer);
diff --git a/src/Analyzer/NameResolver.php b/src/Analyzer/NameResolver.php
index 5842a3d3..47e5a49d 100644
--- a/src/Analyzer/NameResolver.php
+++ b/src/Analyzer/NameResolver.php
@@ -58,7 +58,7 @@ class NameResolver extends NodeVisitorAbstract
      * @param ErrorHandler|null $errorHandler Error handler
      * @param array             $options      Options
      */
-    public function __construct(ErrorHandler $errorHandler = null, array $options = [])
+    public function __construct(?ErrorHandler $errorHandler = null, array $options = [])
     {
         $this->nameContext = new NameContext($errorHandler ?? new ErrorHandler\Throwing());
         $this->preserveOriginalNames = $options['preserveOriginalNames'] ?? false;
@@ -307,7 +307,7 @@ protected function resolveAttrGroups(Node $node): void
         }
     }
 
-    private function addAlias(Stmt\UseUse $use, int $type, Name $prefix = null): void
+    private function addAlias(Stmt\UseUse $use, int $type, ?Name $prefix = null): void
     {
         // Add prefix for group uses
         /** @var Name $name */
diff --git a/src/Expression/ForClasses/NotHaveDependencyOutsideNamespace.php b/src/Expression/ForClasses/NotHaveDependencyOutsideNamespace.php
index 19ac84dd..906131ee 100644
--- a/src/Expression/ForClasses/NotHaveDependencyOutsideNamespace.php
+++ b/src/Expression/ForClasses/NotHaveDependencyOutsideNamespace.php
@@ -18,11 +18,14 @@ class NotHaveDependencyOutsideNamespace implements Expression
     private $namespace;
     /** @var array */
     private $externalDependenciesToExclude;
+    /** @var bool */
+    private $excludeCoreNamespace;
 
-    public function __construct(string $namespace, array $externalDependenciesToExclude = [])
+    public function __construct(string $namespace, array $externalDependenciesToExclude = [], bool $excludeCoreNamespace = false)
     {
         $this->namespace = $namespace;
         $this->externalDependenciesToExclude = $externalDependenciesToExclude;
+        $this->excludeCoreNamespace = $excludeCoreNamespace;
     }
 
     public function describe(ClassDescription $theClass, string $because): Description
@@ -46,6 +49,10 @@ public function evaluate(ClassDescription $theClass, Violations $violations, str
                 continue;
             }
 
+            if ($this->excludeCoreNamespace && '' === $externalDep->getFQCN()->namespace()) {
+                continue;
+            }
+
             $violation = Violation::createWithErrorLine(
                 $theClass->getFQCN(),
                 ViolationMessage::withDescription(
diff --git a/src/Rules/Violation.php b/src/Rules/Violation.php
index f91d2a5e..dd1bfb8f 100644
--- a/src/Rules/Violation.php
+++ b/src/Rules/Violation.php
@@ -15,7 +15,7 @@ class Violation implements \JsonSerializable
     /** @var string */
     private $error;
 
-    public function __construct(string $fqcn, string $error, int $line = null)
+    public function __construct(string $fqcn, string $error, ?int $line = null)
     {
         $this->fqcn = $fqcn;
         $this->error = $error;
diff --git a/tests/E2E/Cli/CheckCommandTest.php b/tests/E2E/Cli/CheckCommandTest.php
index ffc479f1..ec1389a1 100644
--- a/tests/E2E/Cli/CheckCommandTest.php
+++ b/tests/E2E/Cli/CheckCommandTest.php
@@ -148,8 +148,8 @@ public function test_you_can_ignore_the_default_baseline(): void
 
     protected function runCheck(
         $configFilePath = null,
-        bool $stopOnFailure = null,
-        string $useBaseline = null,
+        ?bool $stopOnFailure = null,
+        ?string $useBaseline = null,
         $generateBaseline = false,
         bool $skipBaseline = false
     ): ApplicationTester {
@@ -181,7 +181,7 @@ protected function runCheck(
         return $appTester;
     }
 
-    protected function assertCheckHasErrors(ApplicationTester $applicationTester, string $expectedOutput = null): void
+    protected function assertCheckHasErrors(ApplicationTester $applicationTester, ?string $expectedOutput = null): void
     {
         $this->assertEquals(self::ERROR_CODE, $applicationTester->getStatusCode());
         if (null != $expectedOutput) {
@@ -191,7 +191,7 @@ protected function assertCheckHasErrors(ApplicationTester $applicationTester, st
         }
     }
 
-    protected function assertCheckHasNoErrorsLike(ApplicationTester $applicationTester, string $expectedOutput = null): void
+    protected function assertCheckHasNoErrorsLike(ApplicationTester $applicationTester, ?string $expectedOutput = null): void
     {
         $this->assertEquals(self::ERROR_CODE, $applicationTester->getStatusCode());
         if (null != $expectedOutput) {
diff --git a/tests/Unit/Expressions/ForClasses/NotHaveDependencyOutsideNamespaceTest.php b/tests/Unit/Expressions/ForClasses/NotHaveDependencyOutsideNamespaceTest.php
index 3105c69a..f2322cbf 100644
--- a/tests/Unit/Expressions/ForClasses/NotHaveDependencyOutsideNamespaceTest.php
+++ b/tests/Unit/Expressions/ForClasses/NotHaveDependencyOutsideNamespaceTest.php
@@ -64,7 +64,11 @@ public function test_it_should_return_false_if_depends_on_namespace(): void
         $notHaveDependencyOutsideNamespace = new NotHaveDependencyOutsideNamespace('myNamespace');
         $classDescription = new ClassDescription(
             FullyQualifiedClassName::fromString('HappyIsland'),
-            [new ClassDependency('myNamespace', 100), new ClassDependency('another\class', 200)],
+            [
+                new ClassDependency('myNamespace', 100),
+                new ClassDependency('another\class', 200),
+                new ClassDependency('\DateTime', 300),
+            ],
             [],
             null,
             false,
@@ -77,7 +81,7 @@ public function test_it_should_return_false_if_depends_on_namespace(): void
         $because = 'we want to add this rule for our software';
         $violations = new Violations();
         $notHaveDependencyOutsideNamespace->evaluate($classDescription, $violations, $because);
-        self::assertNotEquals(0, $violations->count());
+        self::assertEquals(2, $violations->count());
     }
 
     public function test_it_should_not_return_violation_error_if_dependency_excluded(): void
@@ -100,4 +104,25 @@ public function test_it_should_not_return_violation_error_if_dependency_excluded
         $notHaveDependencyOutsideNamespace->evaluate($classDescription, $violations, $because);
         self::assertEquals(0, $violations->count());
     }
+
+    public function test_it_should_not_return_violation_error_if_core_dependency_excluded(): void
+    {
+        $notHaveDependencyOutsideNamespace = new NotHaveDependencyOutsideNamespace('myNamespace', [], true);
+        $classDescription = new ClassDescription(
+            FullyQualifiedClassName::fromString('HappyIsland'),
+            [new ClassDependency('\DateTime', 100)],
+            [],
+            null,
+            false,
+            false,
+            false,
+            false,
+            false,
+            false
+        );
+        $because = 'we want to add this rule for our software';
+        $violations = new Violations();
+        $notHaveDependencyOutsideNamespace->evaluate($classDescription, $violations, $because);
+        self::assertEquals(0, $violations->count());
+    }
 }