diff --git a/hw/char/serial.c b/hw/char/serial.c
index fd80ae5592..840da89de7 100644
--- a/hw/char/serial.c
+++ b/hw/char/serial.c
@@ -344,7 +344,7 @@ static void serial_ioport_write(void *opaque, hwaddr addr, uint64_t val,
 {
     SerialState *s = opaque;
 
-    addr &= 7;
+    assert(size == 1 && addr < 8);
     trace_serial_ioport_write(addr, val);
     switch(addr) {
     default:
@@ -485,7 +485,7 @@ static uint64_t serial_ioport_read(void *opaque, hwaddr addr, unsigned size)
     SerialState *s = opaque;
     uint32_t ret;
 
-    addr &= 7;
+    assert(size == 1 && addr < 8);
     switch(addr) {
     default:
     case 0: