index_is_out_of_bounds problem?

Hello,

I habe the following spirit grammar snippet:
I don't know, why the error message index_out_of_bounds appears.

Can You Help
Jens

             rfc_header
                 = tok.kw_http >> '/' >> double_
                               >> ' ' >> int_
                               >> ' ' >> tok.identifier
                       [
                       qi::_val = phx::construct<expression_ast>(
                            std::string("header1")
                           , qi::_3
                           , qi::_5
                           , qi::_7
                           , qi::_val)
                       ]
                 ;

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