diff --git a/src/filesystem/Filesystem.php b/src/filesystem/Filesystem.php
--- a/src/filesystem/Filesystem.php
+++ b/src/filesystem/Filesystem.php
@@ -750,6 +750,13 @@
       $root = realpath($root);
     }
 
+    // NOTE: We don't use `isDescendant()` here because we don't want to reject
+    // paths which don't exist on disk.
+    $root_list = new FileList(array($root));
+    if (!$root_list->contains($path)) {
+      return array();
+    }
+
     $walk = array();
     $parts = explode(DIRECTORY_SEPARATOR, $path);
     foreach ($parts as $k => $part) {
@@ -758,11 +765,7 @@
       }
     }
 
-    if (!self::isDescendant($path, $root)) {
-      return array();
-    }
-
-    while ($parts) {
+    while (true) {
       if (phutil_is_windows()) {
         $next = implode(DIRECTORY_SEPARATOR, $parts);
       } else {
@@ -774,6 +777,10 @@
         break;
       }
 
+      if (!$parts) {
+        break;
+      }
+
       array_pop($parts);
     }
 
diff --git a/src/filesystem/__tests__/FilesystemTestCase.php b/src/filesystem/__tests__/FilesystemTestCase.php
--- a/src/filesystem/__tests__/FilesystemTestCase.php
+++ b/src/filesystem/__tests__/FilesystemTestCase.php
@@ -110,6 +110,18 @@
         dirname(__FILE__).'/data/include_dir.txt/subdir.txt',
         array(),
       ),
+
+      'fictional paths work' => array(
+        '/x/y/z',
+        '/',
+        array(
+          '/x/y/z',
+          '/x/y',
+          '/x',
+          '/',
+        ),
+      ),
+
     );
 
     foreach ($test_cases as $test_case) {