diff --git a/src/Symfony/Component/Routing/Loader/YamlFileLoader.php b/src/Symfony/Component/Routing/Loader/YamlFileLoader.php
index bee7a4a5694c..cd137eea78be 100644
--- a/src/Symfony/Component/Routing/Loader/YamlFileLoader.php
+++ b/src/Symfony/Component/Routing/Loader/YamlFileLoader.php
@@ -171,7 +171,7 @@ protected function parseImport(RouteCollection $collection, array $config, strin
         $schemes = isset($config['schemes']) ? $config['schemes'] : null;
         $methods = isset($config['methods']) ? $config['methods'] : null;
         $trailingSlashOnRoot = $config['trailing_slash_on_root'] ?? true;
-        $namePrefix = $config['name_prefix'] ?? '';
+        $namePrefix = $config['name_prefix'] ?? null;
         $exclude = $config['exclude'] ?? null;
 
         if (isset($config['controller'])) {
diff --git a/src/Symfony/Component/Routing/RouteCollection.php b/src/Symfony/Component/Routing/RouteCollection.php
index a3284771bb57..b661bb13a61d 100644
--- a/src/Symfony/Component/Routing/RouteCollection.php
+++ b/src/Symfony/Component/Routing/RouteCollection.php
@@ -179,8 +179,8 @@ public function addNamePrefix(string $prefix)
 
         foreach ($this->routes as $name => $route) {
             $prefixedRoutes[$prefix.$name] = $route;
-            if (null !== $name = $route->getDefault('_canonical_route')) {
-                $route->setDefault('_canonical_route', $prefix.$name);
+            if (null !== $canonicalName = $route->getDefault('_canonical_route')) {
+                $route->setDefault('_canonical_route', $prefix.$canonicalName);
             }
             if (isset($this->priorities[$name])) {
                 $prefixedPriorities[$prefix.$name] = $this->priorities[$name];
diff --git a/src/Symfony/Component/Routing/Tests/RouteCollectionTest.php b/src/Symfony/Component/Routing/Tests/RouteCollectionTest.php
index 05d3d0162ad1..90a66dbc679e 100644
--- a/src/Symfony/Component/Routing/Tests/RouteCollectionTest.php
+++ b/src/Symfony/Component/Routing/Tests/RouteCollectionTest.php
@@ -363,4 +363,21 @@ public function testAddWithPriority()
 
         $this->assertSame($expected, $collection2->all());
     }
+
+    public function testAddWithPriorityAndPrefix()
+    {
+        $collection3 = new RouteCollection();
+        $collection3->add('foo3', $foo3 = new Route('/foo'), 0);
+        $collection3->add('bar3', $bar3 = new Route('/bar'), 1);
+        $collection3->add('baz3', $baz3 = new Route('/baz'));
+        $collection3->addNamePrefix('prefix_');
+
+        $expected = [
+            'prefix_bar3' => $bar3,
+            'prefix_foo3' => $foo3,
+            'prefix_baz3' => $baz3,
+        ];
+
+        $this->assertSame($expected, $collection3->all());
+    }
 }