Content Search web part - 'Me' Parameter

Hi there,
I'm trying to use a Content Search query to find items where a field value is = to the current logged in user. This filter is usually achieved by using the [Me] param. However it doesn't seem to be working. Here is the query and also a screen grab (see below).

If I change the value inside {Me} to surname, firstname it will return items. 

Any ideas what the syntax should be?  I've also tried {user.name} and this doesn't work either.

Thanks

Alan 

path:"https://~ [path to site]"  IssueAssignedTo:{Me}