diff --git a/src/Composer/Cache.php b/src/Composer/Cache.php
index 61ae44f82..cafad5a28 100644
--- a/src/Composer/Cache.php
+++ b/src/Composer/Cache.php
@@ -162,11 +162,11 @@ class Cache
                     unlink($tempFileName);
 
                     $message = sprintf(
-                        '<warning>Writing %1$s into cache failed after %2$u of %3$u bytes written, only %4$u bytes of free space available</warning>',
+                        '<warning>Writing %1$s into cache failed after %2$u of %3$u bytes written, only %4$s bytes of free space available</warning>',
                         $tempFileName,
                         $m[1],
                         $m[2],
-                        @disk_free_space(dirname($tempFileName))
+                        function_exists('disk_free_space') ? @disk_free_space(dirname($tempFileName)) : 'unknown'
                     );
 
                     $this->io->writeError($message);
diff --git a/src/Composer/Command/DiagnoseCommand.php b/src/Composer/Command/DiagnoseCommand.php
index 242c58154..6e56f3a4f 100644
--- a/src/Composer/Command/DiagnoseCommand.php
+++ b/src/Composer/Command/DiagnoseCommand.php
@@ -370,6 +370,10 @@ EOT
      */
     private function checkDiskSpace(Config $config)
     {
+        if (!function_exists('disk_free_space')) {
+            return true;
+        }
+
         $minSpaceFree = 1024 * 1024;
         if ((($df = @disk_free_space($dir = $config->get('home'))) !== false && $df < $minSpaceFree)
             || (($df = @disk_free_space($dir = $config->get('vendor-dir'))) !== false && $df < $minSpaceFree)
diff --git a/src/Composer/Console/Application.php b/src/Composer/Console/Application.php
index e7c3a32a5..86e89e88b 100644
--- a/src/Composer/Console/Application.php
+++ b/src/Composer/Console/Application.php
@@ -386,7 +386,7 @@ class Application extends BaseApplication
         Silencer::suppress();
         try {
             $composer = $this->getComposer(false, true);
-            if ($composer) {
+            if (null !== $composer && function_exists('disk_free_space')) {
                 $config = $composer->getConfig();
 
                 $minSpaceFree = 1024 * 1024;