diff --git a/src/Composer/Installer.php b/src/Composer/Installer.php
index ba602832c..f1e1f5267 100644
--- a/src/Composer/Installer.php
+++ b/src/Composer/Installer.php
@@ -560,8 +560,14 @@ class Installer
         $constraint->setPrettyString($rootPackage->getPrettyVersion());
         $request->install($rootPackage->getName(), $constraint);
 
+        $installedPackages = $platformRepo->getPackages();
+        if ($this->additionalInstalledRepository) {
+            $additionalInstalledPackages = $this->additionalInstalledRepository->getPackages();
+            $installedPackages = array_merge($installedPackages, $additionalInstalledPackages);
+        }
+
         // fix the version of all platform packages to prevent the solver trying to remove those
-        foreach ($platformRepo->getPackages() as $package) {
+        foreach ($installedPackages as $package) {
             $constraint = new VersionConstraint('=', $package->getVersion());
             $constraint->setPrettyString($package->getPrettyVersion());