barthat on "PHP Warning - how to find cause?"

I am having trouble tracking the source of this PHP warning

PHP Warning: call_user_func_array() expects parameter 1 to be a valid callback, no array or string given in /home/xyz/public_html/mydomain.com/wp-includes/plugin.php on line 199

I am using php 5.4 with error_reporting = "E_ALL & ~E_DEPRECATED & ~E_STRICT".

I think the last plugin I updated was autoptomize but I am not certain and the warning doesn't indicate which plugin is at fault.

Any guidance welcome.