diff --git a/hw/core/sysbus.c b/hw/core/sysbus.c
index 77ab351ce1..294f90b7de 100644
--- a/hw/core/sysbus.c
+++ b/hw/core/sysbus.c
@@ -199,6 +199,7 @@ void sysbus_init_mmio(SysBusDevice *dev, MemoryRegion *memory)
 
 MemoryRegion *sysbus_mmio_get_region(SysBusDevice *dev, int n)
 {
+    assert(n >= 0 && n < QDEV_MAX_MMIO);
     return dev->mmio[n].memory;
 }