diff --git a/core/modules/system/src/Tests/Routing/RouterTest.php b/core/modules/system/src/Tests/Routing/RouterTest.php
index 4d532d1..ce56a72 100644
--- a/core/modules/system/src/Tests/Routing/RouterTest.php
+++ b/core/modules/system/src/Tests/Routing/RouterTest.php
@@ -211,6 +211,27 @@ public function testRouterMatching() {
   }
 
   /**
+   * Tests the route path case insensitivity
+   */
+  public function testRoutePathMixedCase() {
+    $this->drupalGet('router_test/TEST14/1');
+    $this->assertResponse(200);
+    $this->assertText('User route "entity.user.canonical" was matched.');
+
+    $this->drupalGet('/router_test/mixEDCASE2');
+    $this->assertResponse(200);
+    $this->assertRaw('test2', 'The correct string was returned because the route was successful.');
+
+    $this->drupalGet('/router_TEST/MIXEDcase2');
+    $this->assertResponse(200);
+    $this->assertRaw('test2', 'The correct string was returned because the route was successful.');
+
+    $this->drupalGet('/router_test/mixedcase2');
+    $this->assertResponse(200);
+    $this->assertRaw('test2', 'The correct string was returned because the route was successful.');
+  }
+
+  /**
    * Tests that a PSR-7 response works.
    */
   public function testRouterResponsePsr7() {
diff --git a/core/modules/system/tests/modules/router_test_directory/router_test.routing.yml b/core/modules/system/tests/modules/router_test_directory/router_test.routing.yml
index e0c91dd..2318822 100644
--- a/core/modules/system/tests/modules/router_test_directory/router_test.routing.yml
+++ b/core/modules/system/tests/modules/router_test_directory/router_test.routing.yml
@@ -189,3 +189,10 @@ router_test.hierarchy_parent_child2:
     _controller: '\Drupal\router_test\TestControllers::test'
   requirements:
     _access: 'TRUE'
+
+router_test.mixedcase.2:
+  path: '/router_test/mixedCASE2'
+  defaults:
+    _controller: '\Drupal\router_test\TestControllers::test2'
+  requirements:
+    _access: 'TRUE'