diff --git a/src/Message/ServerRequest.php b/src/Message/ServerRequest.php
index 009e4a7..fa32ddc 100644
--- a/src/Message/ServerRequest.php
+++ b/src/Message/ServerRequest.php
@@ -4,6 +4,7 @@ namespace WellRESTed\Message;
 
 use Psr\Http\Message\ServerRequestInterface;
 use Psr\Http\Message\StreamInterface;
+use Psr\Http\Message\UploadedFileInterface;
 
 /**
  * Representation of an incoming, server-side HTTP request.
@@ -193,7 +194,7 @@ class ServerRequest extends Request implements ServerRequestInterface
     {
         if (!$this->isValidUploadedFilesTree($uploadedFiles)) {
             throw new \InvalidArgumentException(
-                "withUploadedFiles expects an array with string keys and UploadedFileInterface[] values");
+                "withUploadedFiles expects an array tree with UploadedFileInterface leaves.");
         }
 
         $request = clone $this;
@@ -475,36 +476,45 @@ class ServerRequest extends Request implements ServerRequestInterface
         return $headers;
     }
 
+    /**
+     * @param array $uploadedFiles
+     * @return bool
+     */
     private function isValidUploadedFilesTree(array $uploadedFiles)
     {
-        // Ensure all keys are strings.
+        // Allow empty array.
+        if (count($uploadedFiles) === 0) {
+            return true;
+        }
+
+        // All keys MUST be strings.
         $keys = array_keys($uploadedFiles);
         if (count($keys) !== count(array_filter($keys, "is_string"))) {
             return false;
         }
 
-        // All values must be UploadedFileInterface[].
+        foreach ($uploadedFiles as $branch) {
+            if (!$this->isValidUploadedFilesBranch($branch)) {
+                return false;
+            }
+        }
 
-        // Ensure all values are arrays.
-        $values = array_values($uploadedFiles);
-        if (count($values) !== count(array_filter($values, "is_array"))) {
+        return true;
+    }
+
+    private function isValidUploadedFilesBranch($branch)
+    {
+        if ($branch instanceof UploadedFileInterface) {
+            return true;
+        }
+
+        if (!is_array($branch)) {
             return false;
         }
 
-        $isUploadedFileInterface = function ($object) {
-            return is_object($object) && in_array('Psr\Http\Message\UploadedFileInterface', class_implements($object));
-        };
-
-        foreach ($values as $items) {
-
-            // Ensure values are list arrays.
-            if (array_keys($items) !== range(0, count($items) - 1)) {
-                return false;
-            }
-
-            // Ensure all items are UploadedFileInterfaces
-            $itemValues = array_values($items);
-            if (count($itemValues) !== count(array_filter($itemValues, $isUploadedFileInterface))) {
+        $values = array_values($branch);
+        foreach ($values as $value) {
+            if (!$this->isValidUploadedFilesBranch($value)) {
                 return false;
             }
         }
diff --git a/test/tests/unit/Message/ServerRequestTest.php b/test/tests/unit/Message/ServerRequestTest.php
index 685d0b5..9653bc6 100644
--- a/test/tests/unit/Message/ServerRequestTest.php
+++ b/test/tests/unit/Message/ServerRequestTest.php
@@ -369,7 +369,7 @@ class ServerRequestTest extends \PHPUnit_Framework_TestCase
     public function testWithUploadedFilesCreatesNewInstance()
     {
         $uploadedFiles = [
-            "file" => [new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0)]
+            "file" => new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0)
         ];
         $request = new ServerRequest();
         $request1 = $request->withUploadedFiles([]);
@@ -380,17 +380,32 @@ class ServerRequestTest extends \PHPUnit_Framework_TestCase
     /**
      * @covers WellRESTed\Message\ServerRequest::withUploadedFiles
      * @covers WellRESTed\Message\ServerRequest::isValidUploadedFilesTree
+     * @dataProvider validUploadedFilesProvider
      */
-    public function testWithUploadedFilesReturnsPassedUploadedFiles()
+    public function testWithUploadedFilesReturnsPassedUploadedFiles($uploadedFiles)
     {
-        $uploadedFiles = [
-            "file" => [new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0)]
-        ];
         $request = new ServerRequest();
         $request = $request->withUploadedFiles($uploadedFiles);
         $this->assertSame($uploadedFiles, $request->getUploadedFiles());
     }
 
+    public function validUploadedFilesProvider()
+    {
+        return [
+            [[]],
+            [["files" => new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0)]],
+            [["nested" => [
+                "level2" => new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0)
+            ]]],
+            [["nestedList" => [
+                "level2" => [
+                    new UploadedFile("file1.html", "text/html", 524, "/tmp/php9hNlHe", 0),
+                    new UploadedFile("file2.html", "text/html", 524, "/tmp/php9hNshj", 0)
+                ]
+            ]]]
+        ];
+    }
+
     /**
      * @covers WellRESTed\Message\ServerRequest::withUploadedFiles
      * @covers WellRESTed\Message\ServerRequest::isValidUploadedFilesTree
@@ -408,33 +423,54 @@ class ServerRequestTest extends \PHPUnit_Framework_TestCase
         return [
             // All keys must be strings
             [[new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0)]],
-
-            // All values must be arrays.
-            [["file" => new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0)]],
-
-            // All values must be list arrays.
             [
-                [
-                    "file" =>
-                        [
-                            "file1" => new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0)
-                        ]
-                ]
+                [new UploadedFile("index1.html", "text/html", 524, "/tmp/php9hNlHe", 0)],
+                [new UploadedFile("index2.html", "text/html", 524, "/tmp/php9hNlHe", 0)]
             ],
             [
-                [
-                    "file" => [
-                        0 => new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0),
-                        2 => new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0)
+                "single" => [
+                    "name" => "single.txt",
+                    "type" => "text/plain",
+                    "tmp_name" => "/tmp/php9hNlHe",
+                    "error" => UPLOAD_ERR_OK,
+                    "size" => 524
+                ],
+                "nested" => [
+                    "level2" => [
+                        "name" => "nested.json",
+                        "type" => "application/json",
+                        "tmp_name" => "/tmp/phpadhjk",
+                        "error" => UPLOAD_ERR_OK,
+                        "size" => 1024
                     ]
-                ]
-            ],
-            [
-                [
-                    "file" => [
-                        new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0),
-                        new UploadedFile("index.html", "text/html", 524, "/tmp/php9hNlHe", 0),
-                        "index.html"
+                ],
+                "nestedList" => [
+                    "level2" => [
+                        "name" => [
+                            0 => "nestedList0.jpg",
+                            1 => "nestedList1.jpg",
+                            2 => ""
+                        ],
+                        "type" => [
+                            0 => "image/jpeg",
+                            1 => "image/jpeg",
+                            2 => ""
+                        ],
+                        "tmp_name" => [
+                            0 => "/tmp/phpjpg0",
+                            1 => "/tmp/phpjpg1",
+                            2 => ""
+                        ],
+                        "error" => [
+                            0 => UPLOAD_ERR_OK,
+                            1 => UPLOAD_ERR_OK,
+                            2 => UPLOAD_ERR_NO_FILE
+                        ],
+                        "size" => [
+                            0 => 256,
+                            1 => 4096,
+                            2 => 0
+                        ]
                     ]
                 ]
             ]