c#: What is wrong with this? C

I'm having trouble finding my error. Here is a definition in structures.h


typedef struct book {
  bank_account_t **accounts;
  transaction_t **transactions;
} book_t;

And here is in functions.c where I include header and try to use the type book_t


#include "structures.h"

void load_book(book_t *book) {

}

But I get this error


functions.c:10:16: error: unknown type name ‘book_t’
 void load_book(book_t *book) {
                ^

c#