Index: tests/tests/user.php
===================================================================
--- tests/tests/user.php	(revision 25017)
+++ tests/tests/user.php	(working copy)
@@ -636,4 +636,13 @@
 		// If this test fails, it will error out for calling the to_array() method on a non-object.
 		$this->assertInstanceOf( 'WP_Error', wp_update_user( array( 'ID' => $user_id ) ) );
 	}
+
+	function test_username_exists() {
+		$user_login = 'foo' . rand( 1, 100 );
+		$user_id    = $this->factory->user->create( compact( 'user_login' ) );
+
+		$this->assertEquals( $user_id, username_exists( $user_login ) );
+		$this->assertNull( username_exists( 'this_should_not_exist' ) );
+	}
+
 }