diff -u b/core/modules/update/src/ProjectCoreCompatibility.php b/core/modules/update/src/ProjectCoreCompatibility.php
--- b/core/modules/update/src/ProjectCoreCompatibility.php
+++ b/core/modules/update/src/ProjectCoreCompatibility.php
@@ -59,31 +59,28 @@
   public function __construct(array $core_data, array $core_releases) {
     if (isset($core_data['existing_version'])) {
       $this->existingCoreVersion = $core_data['existing_version'];
-      $this->possibleCoreUpdateVersions = $this->getPossibleCoreUpdateVersions($core_data['existing_version'], $core_releases);
+      $this->possibleCoreUpdateVersions = $this->getPossibleCoreUpdateVersions($core_releases);
     }
   }
 
   /**
    * Gets the core versions that should be considered for compatibility ranges.
    *
-   * @param string $existing_version
-   *   The existing (currently installed) version of Drupal core.
-   *   @todo No longer needed now that this is a protected member of the class?
    * @param array $core_releases
    *   The Drupal core available releases.
    *
    * @return string[]
    *   The core version numbers that are possible to update the site to.
    */
-  protected function getPossibleCoreUpdateVersions($existing_version, array $core_releases) {
-    if (!isset($core_releases[$existing_version])) {
+  protected function getPossibleCoreUpdateVersions(array $core_releases) {
+    if (!isset($core_releases[$this->existingCoreVersion])) {
       // If we can't determine the existing version of core then we can't
       // calculate the core compatibility of a given release based on core
       // versions after the existing version.
       return [];
     }
     $core_release_versions = array_keys($core_releases);
-    $possible_core_update_versions = Semver::satisfiedBy($core_release_versions, '>= ' . $existing_version);
+    $possible_core_update_versions = Semver::satisfiedBy($core_release_versions, '>= ' . $this->existingCoreVersion);
     $possible_core_update_versions = Semver::sort($possible_core_update_versions);
     $possible_core_update_versions = array_filter($possible_core_update_versions, function ($version) {
       return VersionParser::parseStability($version) === 'stable';