diff --git a/hw/ppc/prep.c b/hw/ppc/prep.c
index b7f459d475..ebee321148 100644
--- a/hw/ppc/prep.c
+++ b/hw/ppc/prep.c
@@ -601,7 +601,7 @@ static int prep_set_cmos_checksum(DeviceState *dev, void *opaque)
     uint16_t checksum = *(uint16_t *)opaque;
     ISADevice *rtc;
 
-    if (object_dynamic_cast(OBJECT(dev), "mc146818rtc")) {
+    if (object_dynamic_cast(OBJECT(dev), TYPE_MC146818_RTC)) {
         rtc = ISA_DEVICE(dev);
         rtc_set_memory(rtc, 0x2e, checksum & 0xff);
         rtc_set_memory(rtc, 0x3e, checksum & 0xff);