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src/Composer/Package/Locker.php

@@ -177,6 +177,8 @@ class Locker
     {
         $lockData = $this->getLockData();
 
+        // return null if not set to allow caller logic to choose the
+        // right behavior since old lock files have no prefer-stable
         return isset($lockData['prefer-stable']) ? $lockData['prefer-stable'] : null;
     }