Fix issue #520
diff --git a/system/helpers/date_helper.php b/system/helpers/date_helper.php
index 077a671..065a223 100644
--- a/system/helpers/date_helper.php
+++ b/system/helpers/date_helper.php
@@ -488,6 +488,10 @@
 		{
 			return 'Unknown';
 		}
+		elseif (empty($format))
+		{
+			$format = 'U';
+		}
 
 		// Date like: YYYYMM
 		if (preg_match('/^\d{6}$/i', $bad_date))
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index 7748f9b..ec33414 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -270,6 +270,7 @@
 -  Fixed a bug in protect_identifiers() where if passed along with the field names, operators got escaped as well.
 -  Fixed a bug (#10) - :doc:`URI Library <libraries/uri>` internal method _detect_uri() failed with paths containing a colon.
 -  Fixed a bug (#1387) - :doc:`Query Builder <database/query_builder>`'s from() method didn't escape table aliases.
+-  Fixed a bug (#520) - :doc:`Date Helper <helpers/date_helper>` function nice_date() failed when the optional second parameter is not passed.
 
 Version 2.1.1
 =============