Error on 'pm on registration' plugin

[theman]

Hello
I get this error on plugin list of admin cp:

Code:

Warning: Call-time pass-by-reference has been deprecated - argument passed by value; If you would like to pass it by reference, modify the declaration of [runtime function name](). If you would like to enable call-time pass-by-reference, you can set allow_call_time_pass_reference to true in your INI file. However, future versions may not support this any longer. in /www/.../inc/plugins/pmonreg.php on line 121

Can you fix this please?
Thank you.