Include struct in the %union def with Bison/Yacc

I am trying to include a struct as part of the union with Bison, but I get an error on the 'struct node args' in %union:

parser.y:17: error: field ‘args’ has incomplete type

The Code:

struct node {
    char * val;
    struct node * next;
};

%}

%union {
    char * string;
    struct node args;
}

%token <string> CD WORD PWD EXIT

%type <args> arg_list

Anyone know what I am doing wrong?