My C code

#include <stdio.h>
#include <stdlib.h>
#include "help.h"

int test(int x, P *ut) {
    int point = 10;

    ut->dt[10].max_x = NULL;
}

int main(int argc, char** argv) {

    return (EXIT_SUCCESS);
}

my help.h file code

typedef struct{
    double max_x;
    double max_y;
}X;

typedef struct{
    X dt[10];
}P;

I got an error i.e

error: incompatible types in assignment

error comes in here

ut->dt[10].max_x = NULL;

can anybody help me. thanks in advance.