diff --git a/hw/char/serial.c b/hw/char/serial.c
index 376bd2f240f3..eb72191ee766 100644
--- a/hw/char/serial.c
+++ b/hw/char/serial.c
@@ -1005,7 +1005,7 @@ static void serial_mm_write(void *opaque, hwaddr addr,
                             uint64_t value, unsigned size)
 {
     SerialState *s = opaque;
-    value &= ~0u >> (32 - (size * 8));
+    value &= 255;
     serial_ioport_write(s, addr >> s->it_shift, value, 1);
 }
 
@@ -1014,16 +1014,22 @@ static const MemoryRegionOps serial_mm_ops[3] = {
         .read = serial_mm_read,
         .write = serial_mm_write,
         .endianness = DEVICE_NATIVE_ENDIAN,
+        .valid.max_access_size = 8,
+        .impl.max_access_size = 8,
     },
     [DEVICE_LITTLE_ENDIAN] = {
         .read = serial_mm_read,
         .write = serial_mm_write,
         .endianness = DEVICE_LITTLE_ENDIAN,
+        .valid.max_access_size = 8,
+        .impl.max_access_size = 8,
     },
     [DEVICE_BIG_ENDIAN] = {
         .read = serial_mm_read,
         .write = serial_mm_write,
         .endianness = DEVICE_BIG_ENDIAN,
+        .valid.max_access_size = 8,
+        .impl.max_access_size = 8,
     },
 };