diff --git a/fixtures/set035-composer-files-autoload/test.php b/fixtures/set035-composer-files-autoload/test.php
index 9db720284..03c636ade 100644
--- a/fixtures/set035-composer-files-autoload/test.php
+++ b/fixtures/set035-composer-files-autoload/test.php
@@ -2,7 +2,7 @@
 
 $output = file_get_contents(__DIR__.'/output');
 
-$functionAutoloadFailed = 0 === preg_match(
+$functionAutoloadFailed = 1 === preg_match(
     '#PHP Fatal error:  Uncaught Error: Call to undefined function GuzzleHttp\\describe_type\(\)#',
     $output,
 );
diff --git a/src/Autoload/ScoperAutoloadGenerator.php b/src/Autoload/ScoperAutoloadGenerator.php
index 90b71f1a9..881131f1b 100644
--- a/src/Autoload/ScoperAutoloadGenerator.php
+++ b/src/Autoload/ScoperAutoloadGenerator.php
@@ -87,8 +87,21 @@ public function dump(): string
                 // scoper-autoload.php @generated by PhpScoper
 
                 namespace {
+                    // Backup the autoloaded Composer files
+                    if (isset(\$GLOBALS['__composer_autoload_files'])) {
+                        \$existingComposerAutoloadFiles = \$GLOBALS['__composer_autoload_files'];
+                    }
+
                     \$loader = require_once __DIR__.'/autoload.php';
+                    // Ensure InstalledVersions is available
                     require_once __DIR__.'/composer/InstalledVersions.php';
+
+                    // Restore the backup
+                    if (isset(\$existingComposerAutoloadFiles)) {
+                        \$GLOBALS['__composer_autoload_files'] = \$existingComposerAutoloadFiles;
+                    } else {
+                        unset(\$GLOBALS['__composer_autoload_files']);
+                    }
                 }
 
                 {$statements}
@@ -104,9 +117,22 @@ public function dump(): string
 
                 // scoper-autoload.php @generated by PhpScoper
 
+                // Backup the autoloaded Composer files
+                if (isset(\$GLOBALS['__composer_autoload_files'])) {
+                    \$existingComposerAutoloadFiles = \$GLOBALS['__composer_autoload_files'];
+                }
+
                 \$loader = require_once __DIR__.'/autoload.php';
+                // Ensure InstalledVersions is available
                 require_once __DIR__.'/composer/InstalledVersions.php';
 
+                // Restore the backup
+                if (isset(\$existingComposerAutoloadFiles)) {
+                    \$GLOBALS['__composer_autoload_files'] = \$existingComposerAutoloadFiles;
+                } else {
+                    unset(\$GLOBALS['__composer_autoload_files']);
+                }
+
                 {$statements}
 
                 return \$loader;
diff --git a/tests/Autoload/ScoperAutoloadGeneratorTest.php b/tests/Autoload/ScoperAutoloadGeneratorTest.php
index a58dfdc81..16ad0c92f 100644
--- a/tests/Autoload/ScoperAutoloadGeneratorTest.php
+++ b/tests/Autoload/ScoperAutoloadGeneratorTest.php
@@ -44,9 +44,22 @@ public static function provideRegistry(): iterable
 
                 // scoper-autoload.php @generated by PhpScoper
 
+                // Backup the autoloaded Composer files
+                if (isset($GLOBALS['__composer_autoload_files'])) {
+                    $existingComposerAutoloadFiles = $GLOBALS['__composer_autoload_files'];
+                }
+
                 $loader = require_once __DIR__.'/autoload.php';
+                // Ensure InstalledVersions is available
                 require_once __DIR__.'/composer/InstalledVersions.php';
 
+                // Restore the backup
+                if (isset($existingComposerAutoloadFiles)) {
+                    $GLOBALS['__composer_autoload_files'] = $existingComposerAutoloadFiles;
+                } else {
+                    unset($GLOBALS['__composer_autoload_files']);
+                }
+
                 return $loader;
 
                 PHP,
@@ -65,9 +78,22 @@ public static function provideRegistry(): iterable
 
                 // scoper-autoload.php @generated by PhpScoper
 
+                // Backup the autoloaded Composer files
+                if (isset($GLOBALS['__composer_autoload_files'])) {
+                    $existingComposerAutoloadFiles = $GLOBALS['__composer_autoload_files'];
+                }
+
                 $loader = require_once __DIR__.'/autoload.php';
+                // Ensure InstalledVersions is available
                 require_once __DIR__.'/composer/InstalledVersions.php';
 
+                // Restore the backup
+                if (isset($existingComposerAutoloadFiles)) {
+                    $GLOBALS['__composer_autoload_files'] = $existingComposerAutoloadFiles;
+                } else {
+                    unset($GLOBALS['__composer_autoload_files']);
+                }
+
                 // Function aliases. For more information see:
                 // https://github.com/humbug/php-scoper/blob/master/docs/further-reading.md#function-aliases
                 if (!function_exists('bar')) { function bar() { return \Humbug\bar(...func_get_args()); } }
@@ -91,9 +117,22 @@ public static function provideRegistry(): iterable
 
                 // scoper-autoload.php @generated by PhpScoper
 
+                // Backup the autoloaded Composer files
+                if (isset($GLOBALS['__composer_autoload_files'])) {
+                    $existingComposerAutoloadFiles = $GLOBALS['__composer_autoload_files'];
+                }
+
                 $loader = require_once __DIR__.'/autoload.php';
+                // Ensure InstalledVersions is available
                 require_once __DIR__.'/composer/InstalledVersions.php';
 
+                // Restore the backup
+                if (isset($existingComposerAutoloadFiles)) {
+                    $GLOBALS['__composer_autoload_files'] = $existingComposerAutoloadFiles;
+                } else {
+                    unset($GLOBALS['__composer_autoload_files']);
+                }
+
                 // Function aliases. For more information see:
                 // https://github.com/humbug/php-scoper/blob/master/docs/further-reading.md#function-aliases
                 if (!function_exists('bar')) { function bar() { return \Humbug\bar(...func_get_args()); } }
@@ -119,8 +158,21 @@ public static function provideRegistry(): iterable
                 // scoper-autoload.php @generated by PhpScoper
 
                 namespace {
+                    // Backup the autoloaded Composer files
+                    if (isset($GLOBALS['__composer_autoload_files'])) {
+                        $existingComposerAutoloadFiles = $GLOBALS['__composer_autoload_files'];
+                    }
+
                     $loader = require_once __DIR__.'/autoload.php';
+                    // Ensure InstalledVersions is available
                     require_once __DIR__.'/composer/InstalledVersions.php';
+
+                    // Restore the backup
+                    if (isset($existingComposerAutoloadFiles)) {
+                        $GLOBALS['__composer_autoload_files'] = $existingComposerAutoloadFiles;
+                    } else {
+                        unset($GLOBALS['__composer_autoload_files']);
+                    }
                 }
 
                 // Function aliases. For more information see:
@@ -156,8 +208,21 @@ public static function provideRegistry(): iterable
                 // scoper-autoload.php @generated by PhpScoper
 
                 namespace {
+                    // Backup the autoloaded Composer files
+                    if (isset($GLOBALS['__composer_autoload_files'])) {
+                        $existingComposerAutoloadFiles = $GLOBALS['__composer_autoload_files'];
+                    }
+
                     $loader = require_once __DIR__.'/autoload.php';
+                    // Ensure InstalledVersions is available
                     require_once __DIR__.'/composer/InstalledVersions.php';
+
+                    // Restore the backup
+                    if (isset($existingComposerAutoloadFiles)) {
+                        $GLOBALS['__composer_autoload_files'] = $existingComposerAutoloadFiles;
+                    } else {
+                        unset($GLOBALS['__composer_autoload_files']);
+                    }
                 }
 
                 // Function aliases. For more information see:
@@ -190,9 +255,22 @@ public static function provideRegistry(): iterable
 
                 // scoper-autoload.php @generated by PhpScoper
 
+                // Backup the autoloaded Composer files
+                if (isset($GLOBALS['__composer_autoload_files'])) {
+                    $existingComposerAutoloadFiles = $GLOBALS['__composer_autoload_files'];
+                }
+
                 $loader = require_once __DIR__.'/autoload.php';
+                // Ensure InstalledVersions is available
                 require_once __DIR__.'/composer/InstalledVersions.php';
 
+                // Restore the backup
+                if (isset($existingComposerAutoloadFiles)) {
+                    $GLOBALS['__composer_autoload_files'] = $existingComposerAutoloadFiles;
+                } else {
+                    unset($GLOBALS['__composer_autoload_files']);
+                }
+
                 // Class aliases. For more information see:
                 // https://github.com/humbug/php-scoper/blob/master/docs/further-reading.md#class-aliases
                 function humbug_phpscoper_expose_class(string $exposed, string $prefixed): void {
@@ -220,9 +298,22 @@ function humbug_phpscoper_expose_class(string $exposed, string $prefixed): void
 
                 // scoper-autoload.php @generated by PhpScoper
 
+                // Backup the autoloaded Composer files
+                if (isset($GLOBALS['__composer_autoload_files'])) {
+                    $existingComposerAutoloadFiles = $GLOBALS['__composer_autoload_files'];
+                }
+
                 $loader = require_once __DIR__.'/autoload.php';
+                // Ensure InstalledVersions is available
                 require_once __DIR__.'/composer/InstalledVersions.php';
 
+                // Restore the backup
+                if (isset($existingComposerAutoloadFiles)) {
+                    $GLOBALS['__composer_autoload_files'] = $existingComposerAutoloadFiles;
+                } else {
+                    unset($GLOBALS['__composer_autoload_files']);
+                }
+
                 // Class aliases. For more information see:
                 // https://github.com/humbug/php-scoper/blob/master/docs/further-reading.md#class-aliases
                 function humbug_phpscoper_expose_class(string $exposed, string $prefixed): void {
@@ -257,8 +348,21 @@ function humbug_phpscoper_expose_class(string $exposed, string $prefixed): void
                 // scoper-autoload.php @generated by PhpScoper
 
                 namespace {
+                    // Backup the autoloaded Composer files
+                    if (isset($GLOBALS['__composer_autoload_files'])) {
+                        $existingComposerAutoloadFiles = $GLOBALS['__composer_autoload_files'];
+                    }
+
                     $loader = require_once __DIR__.'/autoload.php';
+                    // Ensure InstalledVersions is available
                     require_once __DIR__.'/composer/InstalledVersions.php';
+
+                    // Restore the backup
+                    if (isset($existingComposerAutoloadFiles)) {
+                        $GLOBALS['__composer_autoload_files'] = $existingComposerAutoloadFiles;
+                    } else {
+                        unset($GLOBALS['__composer_autoload_files']);
+                    }
                 }
 
                 // Class aliases. For more information see:
@@ -308,9 +412,22 @@ function humbug_phpscoper_expose_class(string $exposed, string $prefixed): void
 
                 // scoper-autoload.php @generated by PhpScoper
 
+                // Backup the autoloaded Composer files
+                if (isset($GLOBALS['__composer_autoload_files'])) {
+                    $existingComposerAutoloadFiles = $GLOBALS['__composer_autoload_files'];
+                }
+
                 $loader = require_once __DIR__.'/autoload.php';
+                // Ensure InstalledVersions is available
                 require_once __DIR__.'/composer/InstalledVersions.php';
 
+                // Restore the backup
+                if (isset($existingComposerAutoloadFiles)) {
+                    $GLOBALS['__composer_autoload_files'] = $existingComposerAutoloadFiles;
+                } else {
+                    unset($GLOBALS['__composer_autoload_files']);
+                }
+
                 // Function aliases. For more information see:
                 // https://github.com/humbug/php-scoper/blob/master/docs/further-reading.md#function-aliases
                 if (!function_exists('__autoload')) { function __autoload($className) { return \Humbug\__autoload(...func_get_args()); } }
@@ -335,8 +452,21 @@ function humbug_phpscoper_expose_class(string $exposed, string $prefixed): void
                 // scoper-autoload.php @generated by PhpScoper
 
                 namespace {
+                    // Backup the autoloaded Composer files
+                    if (isset($GLOBALS['__composer_autoload_files'])) {
+                        $existingComposerAutoloadFiles = $GLOBALS['__composer_autoload_files'];
+                    }
+
                     $loader = require_once __DIR__.'/autoload.php';
+                    // Ensure InstalledVersions is available
                     require_once __DIR__.'/composer/InstalledVersions.php';
+
+                    // Restore the backup
+                    if (isset($existingComposerAutoloadFiles)) {
+                        $GLOBALS['__composer_autoload_files'] = $existingComposerAutoloadFiles;
+                    } else {
+                        unset($GLOBALS['__composer_autoload_files']);
+                    }
                 }
 
                 // Function aliases. For more information see: