diff --git a/UPGRADE-5.0.md b/UPGRADE-5.0.md
index a7312d28e312..abf374dfa04d 100644
--- a/UPGRADE-5.0.md
+++ b/UPGRADE-5.0.md
@@ -477,6 +477,11 @@ Serializer
  * The `AbstractNormalizer::handleCircularReference()` method has two new `$format` and `$context` arguments.
  * Removed support for instantiating a `DataUriNormalizer` with a default MIME type guesser when the `symfony/mime` component isn't installed.
 
+Stopwatch
+---------
+
+ * Removed support for passing `null` as 1st (`$id`) argument of `Section::get()` method, pass a valid child section identifier instead.
+
 Translation
 -----------
 
diff --git a/src/Symfony/Component/Stopwatch/CHANGELOG.md b/src/Symfony/Component/Stopwatch/CHANGELOG.md
index 85a62f018119..14b7dc6dd5ef 100644
--- a/src/Symfony/Component/Stopwatch/CHANGELOG.md
+++ b/src/Symfony/Component/Stopwatch/CHANGELOG.md
@@ -1,6 +1,11 @@
 CHANGELOG
 =========
 
+5.0.0
+-----
+
+ * Removed support for passing `null` as 1st (`$id`) argument of `Section::get()` method, pass a valid child section identifier instead.
+
 4.4.0
 -----
 
diff --git a/src/Symfony/Component/Stopwatch/Section.php b/src/Symfony/Component/Stopwatch/Section.php
index 1bd96bc1edcc..56ff76196e72 100644
--- a/src/Symfony/Component/Stopwatch/Section.php
+++ b/src/Symfony/Component/Stopwatch/Section.php
@@ -56,11 +56,9 @@ public function __construct(float $origin = null, bool $morePrecision = false)
     /**
      * Returns the child section.
      *
-     * @param string $id The child section identifier
-     *
      * @return self|null The child section or null when none found
      */
-    public function get($id)
+    public function get(string $id)
     {
         foreach ($this->children as $child) {
             if ($id === $child->getId()) {
@@ -78,7 +76,7 @@ public function get($id)
      *
      * @return self
      */
-    public function open($id)
+    public function open(?string $id)
     {
         if (null === $id || null === $session = $this->get($id)) {
             $session = $this->children[] = new self(microtime(true) * 1000, $this->morePrecision);
@@ -98,11 +96,9 @@ public function getId()
     /**
      * Sets the session identifier.
      *
-     * @param string $id The session identifier
-     *
      * @return $this
      */
-    public function setId($id)
+    public function setId(string $id)
     {
         $this->id = $id;
 
@@ -112,12 +108,9 @@ public function setId($id)
     /**
      * Starts an event.
      *
-     * @param string      $name     The event name
-     * @param string|null $category The event category
-     *
      * @return StopwatchEvent The event
      */
-    public function startEvent($name, $category)
+    public function startEvent(string $name, ?string $category)
     {
         if (!isset($this->events[$name])) {
             $this->events[$name] = new StopwatchEvent($this->origin ?: microtime(true) * 1000, $category, $this->morePrecision);
@@ -129,11 +122,9 @@ public function startEvent($name, $category)
     /**
      * Checks if the event was started.
      *
-     * @param string $name The event name
-     *
      * @return bool
      */
-    public function isEventStarted($name)
+    public function isEventStarted(string $name)
     {
         return isset($this->events[$name]) && $this->events[$name]->isStarted();
     }
@@ -141,13 +132,11 @@ public function isEventStarted($name)
     /**
      * Stops an event.
      *
-     * @param string $name The event name
-     *
      * @return StopwatchEvent The event
      *
      * @throws \LogicException When the event has not been started
      */
-    public function stopEvent($name)
+    public function stopEvent(string $name)
     {
         if (!isset($this->events[$name])) {
             throw new \LogicException(sprintf('Event "%s" is not started.', $name));
@@ -159,13 +148,11 @@ public function stopEvent($name)
     /**
      * Stops then restarts an event.
      *
-     * @param string $name The event name
-     *
      * @return StopwatchEvent The event
      *
      * @throws \LogicException When the event has not been started
      */
-    public function lap($name)
+    public function lap(string $name)
     {
         return $this->stopEvent($name)->start();
     }
@@ -173,13 +160,11 @@ public function lap($name)
     /**
      * Returns a specific event by name.
      *
-     * @param string $name The event name
-     *
      * @return StopwatchEvent The event
      *
      * @throws \LogicException When the event is not known
      */
-    public function getEvent($name)
+    public function getEvent(string $name)
     {
         if (!isset($this->events[$name])) {
             throw new \LogicException(sprintf('Event "%s" is not known.', $name));
diff --git a/src/Symfony/Component/Stopwatch/Stopwatch.php b/src/Symfony/Component/Stopwatch/Stopwatch.php
index a9f8862f523f..9a3fe56d86e7 100644
--- a/src/Symfony/Component/Stopwatch/Stopwatch.php
+++ b/src/Symfony/Component/Stopwatch/Stopwatch.php
@@ -59,7 +59,7 @@ public function getSections()
      *
      * @throws \LogicException When the section to re-open is not reachable
      */
-    public function openSection($id = null)
+    public function openSection(string $id = null)
     {
         $current = end($this->activeSections);
 
@@ -79,11 +79,9 @@ public function openSection($id = null)
      *
      * @see getSectionEvents()
      *
-     * @param string $id The identifier of the section
-     *
      * @throws \LogicException When there's no started section to be stopped
      */
-    public function stopSection($id)
+    public function stopSection(string $id)
     {
         $this->stop('__section__');
 
@@ -98,12 +96,9 @@ public function stopSection($id)
     /**
      * Starts an event.
      *
-     * @param string      $name     The event name
-     * @param string|null $category The event category
-     *
      * @return StopwatchEvent
      */
-    public function start($name, $category = null)
+    public function start(string $name, string $category = null)
     {
         return end($this->activeSections)->startEvent($name, $category);
     }
@@ -111,11 +106,9 @@ public function start($name, $category = null)
     /**
      * Checks if the event was started.
      *
-     * @param string $name The event name
-     *
      * @return bool
      */
-    public function isStarted($name)
+    public function isStarted(string $name)
     {
         return end($this->activeSections)->isEventStarted($name);
     }
@@ -123,11 +116,9 @@ public function isStarted($name)
     /**
      * Stops an event.
      *
-     * @param string $name The event name
-     *
      * @return StopwatchEvent
      */
-    public function stop($name)
+    public function stop(string $name)
     {
         return end($this->activeSections)->stopEvent($name);
     }
@@ -135,11 +126,9 @@ public function stop($name)
     /**
      * Stops then restarts an event.
      *
-     * @param string $name The event name
-     *
      * @return StopwatchEvent
      */
-    public function lap($name)
+    public function lap(string $name)
     {
         return end($this->activeSections)->stopEvent($name)->start();
     }
@@ -147,11 +136,9 @@ public function lap($name)
     /**
      * Returns a specific event by name.
      *
-     * @param string $name The event name
-     *
      * @return StopwatchEvent
      */
-    public function getEvent($name)
+    public function getEvent(string $name)
     {
         return end($this->activeSections)->getEvent($name);
     }
@@ -159,11 +146,9 @@ public function getEvent($name)
     /**
      * Gets all events for a given section.
      *
-     * @param string $id A section identifier
-     *
      * @return StopwatchEvent[]
      */
-    public function getSectionEvents($id)
+    public function getSectionEvents(string $id)
     {
         return isset($this->sections[$id]) ? $this->sections[$id]->getEvents() : [];
     }