Error C: array type has an incomplete element type

I am trying to compile a program in Ubuntu 11.04 that works well on Windows, but it gives the above error. I added a comment to the line that causes the error. Here is the code:

route_input() {
    int num_routes;//Variable to act as the loop counter for the loop getting route details
    int x;

    char route_id[3];
    char r_source[20];
    char r_destination[20];
    int r_buses;



    printf("Please enter the number of routes used: \n");
    scanf("%d", &num_routes);
    char routes_arr[num_routes][10];//An array to hold the details of each route

    printf("\nNumber of routes is %d\n", num_routes);

    struct route r[num_routes];//An array of structures of type route (This line causes the error)

    fflush(stdin);

    for (x = num_routes; x > 0; x--) {
         printf("\nEnter the route number: ");
         scanf("%s", r[x].route_num);
         printf("Route number is %s", r[x].route_num);


         printf("\nEnter the route source: ");
         fflush(stdin);
         scanf("%s", r[x].source);
         printf("Source = %s", r[x].source);


         printf("\nEnter the route destination: ");
         fflush(stdin);
         gets(r[x].destination);
         printf("Destination = %s", r[x].destination);

         printf("\nEnter the number of buses that use this route: ");
         scanf("%d", &r[x].num_of_buses);
         printf("Number of buses = %d", r[x].num_of_buses);


    }

    for (x = num_routes; x > 0; x--) {
        printf("\n\n+++Routes' Details+++\nRoute number = %s, Source = %s, Destination = %s, Number of buses for this route = %d\n", r[x].route_num, r[x].source, r[x].destination, r[x].num_of_buses);
    }

}