diff --git a/src/Composer/Console/Application.php b/src/Composer/Console/Application.php
index a2165ea76..d6b265f53 100644
--- a/src/Composer/Console/Application.php
+++ b/src/Composer/Console/Application.php
@@ -159,7 +159,7 @@ class Application extends BaseApplication
                 Composer::VERSION,
                 Composer::RELEASE_DATE,
                 defined('HHVM_VERSION') ? 'HHVM '.HHVM_VERSION : 'PHP '.PHP_VERSION,
-                php_uname('s') . ' / ' . php_uname('r')
+                function_exists('php_uname') ? php_uname('s') . ' / ' . php_uname('r') : 'Unknown OS'
             ), true, IOInterface::DEBUG);
 
             if (PHP_VERSION_ID < 50302) {
diff --git a/src/Composer/Util/StreamContextFactory.php b/src/Composer/Util/StreamContextFactory.php
index 9fd28388d..0a6764418 100644
--- a/src/Composer/Util/StreamContextFactory.php
+++ b/src/Composer/Util/StreamContextFactory.php
@@ -143,8 +143,8 @@ final class StreamContextFactory
             $options['http']['header'][] = sprintf(
                 'User-Agent: Composer/%s (%s; %s; %s%s)',
                 Composer::VERSION === '@package_version@' ? 'source' : Composer::VERSION,
-                php_uname('s'),
-                php_uname('r'),
+                function_exists('php_uname') ? php_uname('s') : 'Unknown',
+                function_exists('php_uname') ? php_uname('r') : 'Unknown',
                 $phpVersion,
                 getenv('CI') ? '; CI' : ''
             );