diff -u b/core/includes/file.inc b/core/includes/file.inc
--- b/core/includes/file.inc
+++ b/core/includes/file.inc
@@ -524,7 +524,7 @@
   else {
     // Perhaps $destination is a dir/file?
     $dirname = drupal_dirname($destination);
-    if (!file_prepare_directory($dirname)) {
+    if (!file_prepare_directory($dirname, FILE_CREATE_DIRECTORY)) {
       // The destination is not valid.
       $logger->notice('File %file could not be moved/copied because the destination directory %destination is not configured correctly.', array('%file' => $original_source, '%destination' => $dirname));
       drupal_set_message(t('The specified file %file could not be moved/copied because the destination directory is not properly configured. This may be caused by a problem with file or directory permissions. More information is available in the system log.', array('%file' => $original_source)), 'error');
@@ -574,7 +574,7 @@
   else {
     // Perhaps $destination is a dir/file?
     $dirname = drupal_dirname($destination);
-    if (!file_prepare_directory($dirname, FILE_CREATE_DIRECTORY)) {
+    if (!file_prepare_directory($dirname)) {
       // The destination is not valid.
       $logger->notice('File %file could not be copied because the destination directory %destination is not configured correctly.', array('%file' => $original_source, '%destination' => $dirname));
       drupal_set_message(t('The specified file %file could not be copied because the destination directory is not properly configured. This may be caused by a problem with file or directory permissions. More information is available in the system log.', array('%file' => $original_source)), 'error');
only in patch2:
unchanged:
--- a/core/tests/Drupal/KernelTests/Core/File/UnmanagedCopyTest.php
+++ b/core/tests/Drupal/KernelTests/Core/File/UnmanagedCopyTest.php
@@ -84,6 +84,14 @@ function testOverwriteSelf() {
     $this->assertTrue(file_exists($uri), 'Original file exists after copying onto itself.');
     $this->assertTrue(file_exists($new_filepath), 'Copied file exists after copying onto itself.');
     $this->assertFilePermissions($new_filepath, Settings::get('file_chmod_file', FileSystem::CHMOD_FILE));
+
+    // Copy the file into a subdirectory that does not exist.
+    $new_filepath = file_unmanaged_copy($uri, drupal_dirname($uri) . '/sub/' . drupal_basename($uri), FILE_EXISTS_ERROR);
+    $this->assertTrue($new_filepath, 'Copying into same directory works.');
+    $this->assertNotEqual($new_filepath, $uri, 'Copied file has a new name.');
+    $this->assertTrue(file_exists($uri), 'Original file exists after copying onto itself.');
+    $this->assertTrue(file_exists($new_filepath), 'Copied file exists after copying onto itself.');
+    $this->assertFilePermissions($new_filepath, Settings::get('file_chmod_file', FileSystem::CHMOD_FILE));
   }
 
 }
only in patch2:
unchanged:
--- a/core/tests/Drupal/KernelTests/Core/File/UnmanagedMoveTest.php
+++ b/core/tests/Drupal/KernelTests/Core/File/UnmanagedMoveTest.php
@@ -38,6 +38,15 @@ function testNormal() {
     $this->assertFalse(file_exists($new_filepath), 'No file remains at the old location.');
     $this->assertFilePermissions($newer_filepath, Settings::get('file_chmod_file', FileSystem::CHMOD_FILE));
 
+    // Moving the file into a subdirectory that does not exist.
+    $desired_filepath = 'public://sub/' . $this->randomMachineName();
+    $newest_filepath= file_unmanaged_move($newer_filepath, $desired_filepath, FILE_EXISTS_ERROR);
+    $this->assertTrue($newest_filepath, 'Move was successful.');
+    $this->assertEqual($newest_filepath, $desired_filepath, 'Returned expected filepath.');
+    $this->assertTrue(file_exists($newest_filepath), 'File exists at the new location.');
+    $this->assertFalse(file_exists($newer_filepath), 'No file remains at the old location.');
+    $this->assertFilePermissions($newest_filepath, Settings::get('file_chmod_file', FileSystem::CHMOD_FILE));
+
     // TODO: test moving to a directory (rather than full directory/file path)
     // TODO: test creating and moving normal files (rather than streams)
   }