Error when converting timestamp date

Trying to add date from field (start_date) to @timestamp using the following filter:

match => [ "start_date", "yyyy-MM-dd HH:mm:ss Z" ]
start_date is in the format: 2015-07-26 11:54:43 +0100
I get the following error:
:exception=> "cannot convert instance of class org.jruby.RubyTime to class java.lang.String", :config_parsers=>"yyyy-MM-dd HH:mm:ss Z"
What am i doing wrong?

Please keep it to the one thread - Error when converting Date