diff --git a/src/Symfony/Component/Stopwatch/Section.php b/src/Symfony/Component/Stopwatch/Section.php
index f297a548fce60..d4ce2cc37ef45 100644
--- a/src/Symfony/Component/Stopwatch/Section.php
+++ b/src/Symfony/Component/Stopwatch/Section.php
@@ -58,7 +58,7 @@ public function __construct(float $origin = null, bool $morePrecision = false)
      *
      * @return self|null The child section or null when none found
      */
-    public function get(?string $id)
+    public function get(string $id)
     {
         foreach ($this->children as $child) {
             if ($id === $child->getId()) {
@@ -76,7 +76,7 @@ public function get(?string $id)
      */
     public function open(?string $id)
     {
-        if (null === $session = $this->get($id)) {
+        if (null === $id || null === $session = $this->get($id)) {
             $session = $this->children[] = new self(microtime(true) * 1000, $this->morePrecision);
         }