diff -u b/core/tests/Drupal/KernelTests/Core/Composer/Dependencies/DevDependencyFinderTest.php b/core/tests/Drupal/KernelTests/Core/Composer/Dependencies/DevDependencyFinderTest.php
--- b/core/tests/Drupal/KernelTests/Core/Composer/Dependencies/DevDependencyFinderTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Composer/Dependencies/DevDependencyFinderTest.php
@@ -23,9 +23,50 @@
     // Data provider methods are run before setUp(), so they can't use
     // constants from *.inc files. REQUIREMENT_OK = 0, REQUIREMENT_WARNING = 1.
     return [
-      [0, [], []],
-      [0, ['any_requirement' => '0.0.1'], []],
-      [1, ['dev_requirement' => '0.0.1'], ['dev_requirement' => '0.0.*']],
+      // Tests data for a case when vendor dir is located under app root.
+      [
+        'expected' => 0,
+        'app_root' => '/var/www/drupal/web',
+        'vendor_dir' => '/var/www/drupal/web/vendor',
+        'installed' => [],
+        'dev_requirements' => []
+      ],
+      [
+        'expected' => 0,
+        'app_root' => '/var/www/drupal/web',
+        'vendor_dir' => '/var/www/drupal/web/vendor',
+        'installed' => ['any_requirement' => '0.0.1'],
+        'dev_requirements' => []
+      ],
+      [
+        'expected' => 1,
+        'app_root' => '/var/www/drupal/web',
+        'vendor_dir' => '/var/www/drupal/web/vendor',
+        'installed' => ['dev_requirement' => '0.0.1'],
+        'dev_requirements' => ['dev_requirement' => '0.0.*']
+      ],
+      // Tests data for a case when vendor dir is located outside app root.
+      [
+        'expected' => 0,
+        'app_root' => '/var/www/drupal/web',
+        'vendor_dir' => '/var/www/drupal/vendor',
+        'installed' => [],
+        'dev_requirements' => []
+      ],
+      [
+        'expected' => 0,
+        'app_root' => '/var/www/drupal/web',
+        'vendor_dir' => '/var/www/drupal/vendor',
+        'installed' => ['any_requirement' => '0.0.1'],
+        'dev_requirements' => []
+      ],
+      [
+        'expected' => 0,
+        'app_root' => '/var/www/drupal/web',
+        'vendor_dir' => '/var/www/drupal/vendor',
+        'installed' => ['dev_requirement' => '0.0.1'],
+        'dev_requirements' => ['dev_requirement' => '0.0.*']
+      ],
     ];
   }
 
@@ -33,14 +74,20 @@
    * @covers ::buildRequirements
    * @dataProvider provideBuildRequirements
    */
-  public function testBuildRequirements($expected, $installed, $dev_requirements) {
+  public function testBuildRequirements($expected, $app_root, $vendor_dir, $installed, $dev_requirements) {
     $mock_root = $this->getMockBuilder(ProjectRootInterface::class)
-      ->setMethods(['getInstalledPackages', 'getDevRequirements'])
+      ->setMethods(['getProjectRootDirectory', 'getProjectVendorDirectory', 'getInstalledPackages', 'getDevRequirements'])
       ->getMockForAbstractClass();
     $mock_root->expects($this->once())
+      ->method('getProjectRootDirectory')
+      ->willReturn($app_root);
+    $mock_root->expects($this->once())
+      ->method('getProjectVendorDirectory')
+      ->willReturn($vendor_dir);
+    $mock_root->expects($this->atMost(1))
       ->method('getInstalledPackages')
       ->willReturn($installed);
-    $mock_root->expects($this->once())
+    $mock_root->expects($this->atMost(1))
       ->method('getDevRequirements')
       ->willReturn($dev_requirements);