diff --git a/core/lib/Drupal/Core/Extension/InfoParserDynamic.php b/core/lib/Drupal/Core/Extension/InfoParserDynamic.php
index 2e1dae399d..76628913c6 100644
--- a/core/lib/Drupal/Core/Extension/InfoParserDynamic.php
+++ b/core/lib/Drupal/Core/Extension/InfoParserDynamic.php
@@ -73,9 +73,6 @@ public function parse($filename) {
           throw new InfoParserException("The 'core' or the 'core_version_requirement' key must be present in " . $filename);
         }
       }
-      if (isset($parsed_info['core']) && !preg_match("/^\d\.x$/", $parsed_info['core'])) {
-        throw new InfoParserException("Invalid 'core' value \"{$parsed_info['core']}\" in " . $filename);
-      }
       if (isset($parsed_info['core_version_requirement'])) {
         $supports_pre_core_version_requirement_version = static::isConstraintSatisfiedByPreviousVersion($parsed_info['core_version_requirement'], static::FIRST_CORE_VERSION_REQUIREMENT_SUPPORTED_VERSION);
         // If the 'core_version_requirement' constraint does not satisfy any
@@ -94,6 +91,17 @@ public function parse($filename) {
           throw new InfoParserException("The 'core_version_requirement' can not be used to specify compatibility for a specific version before " . static::FIRST_CORE_VERSION_REQUIREMENT_SUPPORTED_VERSION . " in $filename");
         }
       }
+      if (isset($parsed_info['core'])) {
+        if (!preg_match("/^\d\.x$/", $parsed_info['core'])) {
+          throw new InfoParserException("Invalid 'core' value \"{$parsed_info['core']}\" in " . $filename);
+        }
+        if ($parsed_info['core'] === '9.x') {
+          if (!isset($parsed_info['core_version_requirement'])) {
+            throw new InfoParserException("'core: 9.x' is not supported. Use 'core_version_requirement' to specify core compatibility in " . $filename);
+          }
+          throw new InfoParserException("'core: 9.x' is not supported. 'core_version_requirement' is used to specify core compatibility in " . $filename);
+        }
+      }
 
       // Determine if the extension is compatible with the current version of
       // Drupal core.
diff --git a/core/tests/Drupal/Tests/Core/Extension/InfoParserUnitTest.php b/core/tests/Drupal/Tests/Core/Extension/InfoParserUnitTest.php
index 2e8c9ce1ca..d33d4dea0f 100644
--- a/core/tests/Drupal/Tests/Core/Extension/InfoParserUnitTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/InfoParserUnitTest.php
@@ -274,6 +274,86 @@ public function testInvalidCore() {
     }
   }
 
+  /**
+   * Tests 'core: 9.x' which is not needed.
+   *
+   * @covers ::parse
+   */
+  public function testNotNeededCore() {
+    $not_needed_core = <<<NOT_NEEDED_CORE
+# info.yml for testing invalid core key.
+core: 9.x
+core_version_requirement: ^8 || ^9
+version: VERSION
+type: module
+name: Llama or Alpaca
+description: Tells whether an image is of a Llama or Alpaca
+dependencies:
+  - llama_detector
+  - alpaca_detector
+NOT_NEEDED_CORE;
+
+    vfsStream::setup('modules');
+    vfsStream::create([
+      'fixtures' => [
+        'not_needed_core.info.txt' => $not_needed_core,
+        'not_needed_core-duplicate.info.txt' => $not_needed_core,
+      ],
+    ]);
+    $exception_message = "'core: 9.x' is not supported. 'core_version_requirement' is used to specify core compatibility in vfs://modules/fixtures/not_needed_core%s.info.txt";
+    // Set the expected exception for the 2nd call to parse().
+    $this->expectException('\Drupal\Core\Extension\InfoParserException');
+    $this->expectExceptionMessage(sprintf($exception_message, '-duplicate'));
+
+    try {
+      $this->infoParser->parse(vfsStream::url('modules/fixtures/not_needed_core.info.txt'));
+    }
+    catch (InfoParserException $exception) {
+      $this->assertSame(sprintf($exception_message, ''), $exception->getMessage());
+
+      $this->infoParser->parse(vfsStream::url('modules/fixtures/not_needed_core-duplicate.info.txt'));
+    }
+  }
+
+  /**
+   * Tests a invalid 'core' key.
+   *
+   * @covers ::parse
+   */
+  public function testCore9x() {
+    $not_needed_core = <<<NOT_NEEDED_CORE
+core: 9.x
+version: VERSION
+type: module
+name: Llama or Alpaca
+description: Tells whether an image is of a Llama or Alpaca
+dependencies:
+  - llama_detector
+  - alpaca_detector
+NOT_NEEDED_CORE;
+
+    vfsStream::setup('modules');
+    vfsStream::create([
+      'fixtures' => [
+        'not_needed_core.info.txt' => $not_needed_core,
+        'not_needed_core-duplicate.info.txt' => $not_needed_core,
+      ],
+    ]);
+    $exception_message = "'core: 9.x' is not supported. Use 'core_version_requirement' to specify core compatibility in vfs://modules/fixtures/not_needed_core%s.info.txt";
+    // Set the expected exception for the 2nd call to parse().
+    $this->expectException('\Drupal\Core\Extension\InfoParserException');
+    $this->expectExceptionMessage(sprintf($exception_message, '-duplicate'));
+
+    try {
+      $this->infoParser->parse(vfsStream::url('modules/fixtures/not_needed_core.info.txt'));
+    }
+    catch (InfoParserException $exception) {
+      $this->assertSame(sprintf($exception_message, ''), $exception->getMessage());
+
+      $this->infoParser->parse(vfsStream::url('modules/fixtures/not_needed_core-duplicate.info.txt'));
+    }
+  }
+
   /**
    * Tests a invalid 'core_version_requirement'.
    *