Segmentation fault chapter 6 mpc

[AllanRegush]

Getting Segmentation fault on chapter 6 using mpc d59264a

In the book

mpca_lang(MPCA_LANG_DEFAULT,
  "                                                     \
    number   : /-?[0-9]+/ ;                             \
    operator : '+' | '-' | '*' | '/' ;                  \
    expr     : <number> | '(' <operator> <expr>+ ')' ;  \
    lispy    : /^/ <operator> <expr>+ /$/ ;             \
  ",
  Number, Operator, Expr, Lispy);

As per mpc documentation

mpca_lang(MPCA_LANG_DEFAULT,
  " expression : <product> (('+' | '-') <product>)*; "
  " product    : <value>   (('*' | '/')   <value>)*; "
  " value      : /[0-9]+/ | '(' <expression> ')';    "
  " maths      : /^/ <expression> /$/;               ",
  Expr, Prod, Value, Maths, NULL);

By adding NULL to the parameters this solves this issue.