compiler qi [ ... ] issue

Hello,

why does this peace of code brings the compiler to fail?
when [ .. ] delete, I get no errors/messages, and compile is fine.

TIA
Jens

             symbol_alpha =
                  (qi::char_("a-zA-Z_") >>
                 *(qi::char_("a-zA-Z0-9_"))
                 [
                     qi::_val =
                     phoenix::construct<compile_op>(
phoenix::construct<compile_op>(qi::_1))
                 ]    )
                ;

rule<Iterator,std::string()> symbol_alpha;


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