diff --git a/src/Flex.php b/src/Flex.php
index fdaaf003..35476fc7 100644
--- a/src/Flex.php
+++ b/src/Flex.php
@@ -497,7 +497,7 @@ public function finish(string $rootDir, ?string $originalComposerJsonHash = null
 
     private function synchronizePackageJson(string $rootDir)
     {
-        if (!($this->composer->getPackage()->getExtra()['symfony/flex']['synchronize_package_json'] ?? true)) {
+        if (!filter_var($this->composer->getPackage()->getExtra()['symfony/flex']['synchronize_package_json'] ?? true, \FILTER_VALIDATE_BOOLEAN)) {
             $this->io->writeError('<info>Skip synchronizing package.json with PHP packages</>');
 
             return;