CQ.

composer.json

@@ -35,6 +35,7 @@
     "dnoegel/php-xdg-base-dir": "^0.1.0",
     "doctrine/cache": "^1.8",
     "guzzlehttp/guzzle": "^6.3",
+    "guzzlehttp/psr7": "^1.6",
     "kevinrob/guzzle-cache-middleware": "^3.2",
     "ocramius/package-versions": "^1.4",
     "pimple/pimple": "^3.2",

src/Builder/Builder.php

@@ -152,10 +152,10 @@ class Builder implements BuilderInterface, LoggerAwareInterface
 
             $sourceStream = StreamWrapper::getResource($response->getBody());
             try {
-                $destStream = fopen($dest, 'wb');
+                $destStream = \Safe\fopen($dest, 'wb');
                 try {
-                    $n = \Safe\stream_copy_to_stream($sourceStream, $destStream);
-                    $this->logger->debug("received $n bytes from $source");
+                    $size = \Safe\stream_copy_to_stream($sourceStream, $destStream);
+                    $this->logger->debug("received $size bytes from $source");
                 } finally {
                     fclose($destStream);
                 }