diff --git a/src/applications/people/xaction/PhabricatorUserUsernameTransaction.php b/src/applications/people/xaction/PhabricatorUserUsernameTransaction.php
index 338b296335..e30a131cff 100644
--- a/src/applications/people/xaction/PhabricatorUserUsernameTransaction.php
+++ b/src/applications/people/xaction/PhabricatorUserUsernameTransaction.php
@@ -1,112 +1,121 @@
 <?php
 
 final class PhabricatorUserUsernameTransaction
   extends PhabricatorUserTransactionType {
 
   const TRANSACTIONTYPE = 'user.rename';
 
   public function generateOldValue($object) {
     return $object->getUsername();
   }
 
   public function generateNewValue($object, $value) {
     return $value;
   }
 
   public function applyInternalEffects($object, $value) {
     $object->setUsername($value);
   }
 
   public function applyExternalEffects($object, $value) {
     $actor = $this->getActor();
     $user = $object;
 
     $old_username = $this->getOldValue();
     $new_username = $this->getNewValue();
 
     // The SSH key cache currently includes usernames, so dirty it. See T12554
     // for discussion.
     PhabricatorAuthSSHKeyQuery::deleteSSHKeyCache();
 
     id(new PhabricatorPeopleUsernameMailEngine())
       ->setSender($actor)
       ->setRecipient($object)
       ->setOldUsername($old_username)
       ->setNewUsername($new_username)
       ->sendMail();
   }
 
   public function getTitle() {
     return pht(
       '%s renamed this user from %s to %s.',
       $this->renderAuthor(),
       $this->renderOldValue(),
       $this->renderNewValue());
   }
 
   public function getTitleForFeed() {
     return pht(
       '%s renamed %s from %s to %s.',
       $this->renderAuthor(),
       $this->renderObject(),
       $this->renderOldValue(),
       $this->renderNewValue());
   }
 
   public function validateTransactions($object, array $xactions) {
     $actor = $this->getActor();
     $errors = array();
 
     foreach ($xactions as $xaction) {
       $new = $xaction->getNewValue();
       $old = $xaction->getOldValue();
 
       if ($old === $new) {
         continue;
       }
 
       if (!$actor->getIsAdmin()) {
         $errors[] = $this->newInvalidError(
           pht('You must be an administrator to rename users.'));
       }
 
       if (!strlen($new)) {
-        $errors[] = $this->newRequiredError(
-          pht('New username is required.'),  $xaction);
+        $errors[] = $this->newInvalidError(
+          pht('New username is required.'),
+          $xaction);
       } else if (!PhabricatorUser::validateUsername($new)) {
         $errors[] = $this->newInvalidError(
-          PhabricatorUser::describeValidUsername(), $xaction);
+          PhabricatorUser::describeValidUsername(),
+          $xaction);
       }
 
       $user = id(new PhabricatorPeopleQuery())
         ->setViewer(PhabricatorUser::getOmnipotentUser())
         ->withUsernames(array($new))
         ->executeOne();
-
       if ($user) {
-        $errors[] = $this->newInvalidError(
-          pht('Another user already has that username.'), $xaction);
+        // See T13446. We may be changing the letter case of a username, which
+        // is a perfectly fine edit.
+        $is_self = ($user->getPHID() === $object->getPHID());
+        if (!$is_self) {
+          $errors[] = $this->newInvalidError(
+            pht(
+              'Another user already has the username "%s".',
+              $new),
+            $xaction);
+        }
       }
 
     }
 
     return $errors;
   }
 
   public function getRequiredCapabilities(
     $object,
     PhabricatorApplicationTransaction $xaction) {
 
     // Unlike normal user edits, renames require admin permissions, which
     // is enforced by validateTransactions().
 
     return null;
   }
 
   public function shouldTryMFA(
     $object,
     PhabricatorApplicationTransaction $xaction) {
     return true;
   }
 
 }