I have problems reading from memory from a STRUCT

Code:

format PE GUI 4.0

include 'include\win32a.inc'

section '.code' code readable executable

invoke GetLocalTime, _mytime

invoke wsprintf, _buffer, _prefix, [_mytime.wHour]

invoke MessageBox, 0, _buffer, 0, 0
invoke ExitProcess, 0

section '.data' data readable writeable
_mytime  SYSTEMTIME
_prefix  db "%d", 0
_buffer: times 64 db 0

data import

 library kernel32,'KERNEL32.DLL',\
    user32,'USER32.DLL'

 import user32,\
     wsprintf,'wsprintfA',\
   MessageBox,'MessageBoxA'

 import kernel32,\
        GetLocalTime,'GetLocalTime',\
    ExitProcess,'ExitProcess'

end data

I have problems reading from memory from the struct SYSTEMTIME.
Error is:
D:\fasmc156>fasm keygen.fasm
flat assembler version 1.56
keygen.fasm [9]:
invoke wsprintf, _buffer, _prefix, [_mytime.wHour]
include\macro/stdcall.inc [81] invoke [3]:
pushd arg
error: invalid size of operand.