diff --git a/src/Message/ServerRequest.php b/src/Message/ServerRequest.php
index 6697638..f7e6e62 100644
--- a/src/Message/ServerRequest.php
+++ b/src/Message/ServerRequest.php
@@ -485,48 +485,44 @@ class ServerRequest extends Request implements ServerRequestInterface
     }
 
     /**
-     * @param array $uploadedFiles
+     * @param array $root
      * @return bool
      */
-    private function isValidUploadedFilesTree(array $uploadedFiles)
+    private function isValidUploadedFilesTree(array $root)
     {
         // Allow empty array.
-        if (count($uploadedFiles) === 0) {
+        if (count($root) === 0) {
             return true;
         }
 
-        // All keys MUST be strings.
-        $keys = array_keys($uploadedFiles);
+        // If not empty, the array MUST have all string keys.
+        $keys = array_keys($root);
         if (count($keys) !== count(array_filter($keys, "is_string"))) {
             return false;
         }
 
-        foreach ($uploadedFiles as $branch) {
+        // Valid if each child branch is valid.
+        foreach ($root as $branch) {
             if (!$this->isValidUploadedFilesBranch($branch)) {
                 return false;
             }
         }
-
         return true;
     }
 
     private function isValidUploadedFilesBranch($branch)
     {
-        if ($branch instanceof UploadedFileInterface) {
-            return true;
-        }
-
-        if (!is_array($branch)) {
-            return false;
-        }
-
-        $values = array_values($branch);
-        foreach ($values as $value) {
-            if (!$this->isValidUploadedFilesBranch($value)) {
-                return false;
+        if (is_array($branch)) {
+            // Branch.
+            foreach ($branch as $child) {
+                if (!$this->isValidUploadedFilesBranch($child)) {
+                    return false;
+                }
             }
+            return true;
+        } else {
+            // Leaf. Valid only if this is an UploadedFileInterface.
+            return $branch instanceof UploadedFileInterface;
         }
-
-        return true;
     }
 }
diff --git a/test/tests/unit/Message/ServerRequestTest.php b/test/tests/unit/Message/ServerRequestTest.php
index b1c6134..25b3f35 100644
--- a/test/tests/unit/Message/ServerRequestTest.php
+++ b/test/tests/unit/Message/ServerRequestTest.php
@@ -447,6 +447,7 @@ class ServerRequestTest extends \PHPUnit_Framework_TestCase
     /**
      * @covers ::withUploadedFiles
      * @covers ::isValidUploadedFilesTree
+     * @covers ::isValidUploadedFilesBranch
      * @expectedException \InvalidArgumentException
      * @dataProvider invalidUploadedFilesProvider
      */
@@ -481,7 +482,9 @@ class ServerRequestTest extends \PHPUnit_Framework_TestCase
                         "error" => UPLOAD_ERR_OK,
                         "size" => 1024
                     ]
-                ],
+                ]
+            ],
+            [
                 "nestedList" => [
                     "level2" => [
                         "name" => [