Fatal error: undefined function is_writeable_ACLSafe()

Hi, I tried to enable both your stable and development versions of your plugin, but they both give me the error as follows:

Fatal error: Call to undefined function is_writeable_ACLSafe() in …/wp-content/plugins/prankzapper/wp/pzap_functions.php on line 14

http://wordpress.org/extend/plugins/prankzapper/