Difference between sizeof(empty struct) and sizeof(struct with empty array)?

I have two structs defined as follows:

struct EmptyStruct{

};

struct StructEmptyArr{
    int arr[0];
};

int main(void){
    printf("sizeof(EmptyStruct) = %ld\n", sizeof(EmptyStruct));
    printf("sizeof(StructEmptyArr) = %ld\n", sizeof(StructEmptyArr));

    return 0;
}

Compiled with gcc (g++) 4.8.4 on Ubuntu 14.04, x64.

Output (for both gcc and g++):

sizeof(EmptyStruct) = 1
sizeof(StructEmptyArr) = 0

I can understand why sizeof(EmptyStruct) equals to 1 but cannot understand why sizeof(StructEmptyArr) equals to 0. Why are there differences between two?