diff --git a/src/Composer/Installer/PluginInstaller.php b/src/Composer/Installer/PluginInstaller.php
index bae1d938b..3a982f13b 100644
--- a/src/Composer/Installer/PluginInstaller.php
+++ b/src/Composer/Installer/PluginInstaller.php
@@ -50,7 +50,7 @@ class PluginInstaller extends LibraryInstaller
     {
         // fail install process early if it is going to fail due to a plugin not being allowed
         if (($type === 'install' || $type === 'update') && !$this->getPluginManager()->arePluginsDisabled('local')) {
-            $this->getPluginManager()->isPluginAllowed($package->getName(), false, $package->getExtra()['plugin-optional'] ?? false);
+            $this->getPluginManager()->isPluginAllowed($package->getName(), false, true === ($package->getExtra()['plugin-optional'] ?? false));
         }
 
         return parent::prepare($type, $package, $prevPackage);
diff --git a/src/Composer/Plugin/PluginManager.php b/src/Composer/Plugin/PluginManager.php
index a6c662c84..14eb40211 100644
--- a/src/Composer/Plugin/PluginManager.php
+++ b/src/Composer/Plugin/PluginManager.php
@@ -187,7 +187,7 @@ class PluginManager
             }
         }
 
-        if (!$this->isPluginAllowed($package->getName(), $isGlobalPlugin, (bool) ($package->getExtra()['plugin-optional'] ?? false))) {
+        if (!$this->isPluginAllowed($package->getName(), $isGlobalPlugin, true === ($package->getExtra()['plugin-optional'] ?? false))) {
             $this->io->writeError('Skipped loading "'.$package->getName() . '" '.($isGlobalPlugin || $this->runningInGlobalDir ? '(installed globally) ' : '').'as it is not in config.allow-plugins', true, IOInterface::DEBUG);
 
             return;
@@ -370,7 +370,7 @@ class PluginManager
 
         if ($sourcePackage === null) {
             trigger_error('Calling PluginManager::addPlugin without $sourcePackage is deprecated, if you are using this please get in touch with us to explain the use case', E_USER_DEPRECATED);
-        } elseif (!$this->isPluginAllowed($sourcePackage->getName(), $isGlobalPlugin, (bool) ($sourcePackage->getExtra()['plugin-optional'] ?? false))) {
+        } elseif (!$this->isPluginAllowed($sourcePackage->getName(), $isGlobalPlugin, true === ($sourcePackage->getExtra()['plugin-optional'] ?? false))) {
             $this->io->writeError('Skipped loading "'.get_class($plugin).' from '.$sourcePackage->getName() . '" '.($isGlobalPlugin || $this->runningInGlobalDir ? '(installed globally) ' : '').' as it is not in config.allow-plugins', true, IOInterface::DEBUG);
 
             return;