Some others have reported this error in the forum, http://forum.dokuwiki.org/thread/431
. I haven't been able to duplicate it. Can you give some details of your server setup (OS, Webserver, PHP) and what you are doing that triggers the error.
As I say in the forum, I can see one execution track where its possible to not have an array by line 318. However that execution track should indicate a more serious problem - DokuWiki unable to instantiate its lexer.
Given the error your seeing is only a warning, you may get some success by turning off PHP's display errors and/or altering the reported error level to E_ERROR.