Openerp custom report error - can only parse strings

[Dqexawohmkrnu]

I created openerp custom report, but when I try to open it, I'm getting this error:can only parse strings\[code\](<type 'exceptions.ValueError'>, ValueError(u'can only parse strings',), <traceback object at 0x28c908c0>)\[/code\]Does anyone know what that means and where could be the problem? I read that it may be something wrong with rml file name or path to it. I checked it, but it seems its correct.