diff --git a/core/modules/system/src/Tests/Routing/RouterTest.php b/core/modules/system/src/Tests/Routing/RouterTest.php
index c92a79a..6cbab84 100644
--- a/core/modules/system/src/Tests/Routing/RouterTest.php
+++ b/core/modules/system/src/Tests/Routing/RouterTest.php
@@ -212,7 +212,7 @@ public function testRouterMatching() {
   }
 
   /**
-   * Tests the route path case insensitivity.
+   * Tests the route path and route path with slug case insensitivity.
    */
   public function testRoutePathMixedCase() {
     $this->drupalGet('router_test/TEST14/1');
@@ -230,6 +230,23 @@ public function testRoutePathMixedCase() {
     $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/test3/{mixedCASE}');
+    $this->assertResponse(200);
+    $this->assertRaw('{mixedCASE}', 'The correct string was returned because the route was successful.');
+
+    $this->drupalGet('router_test/test3/{mixedcase}');
+    $this->assertResponse(200);
+    $this->assertRaw('{mixedcase}', 'The correct string was returned because the route was successful.');
+
+    $this->drupalGet('router_test/test3/{mixedCAse}');
+    $this->assertResponse(200);
+    $this->assertRaw('{mixedCAse}', 'The correct string was returned because the route was successful.');
+
+    $this->drupalGet('router_test/test3/{MIXEDCASE}');
+    $this->assertResponse(200);
+    $this->assertRaw('{MIXEDCASE}', 'The correct string was returned because the route was successful.');
+
   }
 
   /**
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 2318822..5fcd5db 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
@@ -196,3 +196,10 @@ router_test.mixedcase.2:
     _controller: '\Drupal\router_test\TestControllers::test2'
   requirements:
     _access: 'TRUE'
+
+router_test.mixedcaseslug.3:
+    path: '/router_test/test3/{mixedCASE}'
+    defaults:
+      _controller: '\Drupal\router_test\TestControllers::test3'
+    requirements:
+      _access: 'TRUE'