Segmentation fault when switching system call the "int" to "syscall" in x86 assembly linux

When I change the system call from int $0x80 to syscall or sysenter in this code:

    mov $4, %rax
    mov $1, %rbx
    mov $String1, %rcx
    mov $16, %rdx
    int $0x80

The program ends in segmentation fault. In GDB it shows:

  

Program received signal SIGSEGV, Segmentation fault.   0x00000000f7ffdbe9 in ?? ()

Why is this change causing this impact? Thanks