diff --git a/system/libraries/Cache/drivers/Cache_file.php b/system/libraries/Cache/drivers/Cache_file.php
index 68bc1ec..c046f3b 100644
--- a/system/libraries/Cache/drivers/Cache_file.php
+++ b/system/libraries/Cache/drivers/Cache_file.php
@@ -267,7 +267,7 @@
 	 */
 	protected function _get($id)
 	{
-		if ( ! file_exists($this->_cache_path.$id))
+		if ( ! is_file($this->_cache_path.$id))
 		{
 			return FALSE;
 		}
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index 00d992a..54dcea8 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -23,6 +23,7 @@
 -  Fixed a bug (#4066) - :doc:`Cache Library <libraries/caching>` couldn't fallback to a backup driver if the primary one is Memcache(d) or Redis.
 -  Fixed a bug (#4073) - :doc:`Email Library <libraries/email>` method ``send()`` could return TRUE in case of an actual failure when an SMTP command fails.
 -  Fixed a bug (#4086) - :doc:`Query Builder <database/query_builder>` didn't apply *dbprefix* to LIKE conditions if the pattern included spaces.
+-  Fixed a bug (#4091) - :doc:`Cache Library <libraries/caching>` 'file' driver could be tricked into accepting empty cache item IDs.
 
 Version 3.0.1
 =============