Inconsistent result from assignment the negative double value to an enum-type variable

double_to_enum.c

#include <stdio.h>

typedef enum {
    ZERO
} NUM;

int main(void) {
    double a = -1.0;
    NUM b = a;

    printf("%d %d\n", b == ZERO, b != ZERO);    // same result !?

    return 0;
}