diff --git a/sys/amd64/amd64/mp_machdep.c b/sys/amd64/amd64/mp_machdep.c
index de6e06d5ed1..69c5c0f4dfd 100644
--- a/sys/amd64/amd64/mp_machdep.c
+++ b/sys/amd64/amd64/mp_machdep.c
@@ -198,7 +198,7 @@ mp_bootaddress(u_int basemem)
 
 	bootMP_size = mptramp_end - mptramp_start;
 	boot_address = trunc_page(basemem * 1024); /* round down to 4k boundary */
-	if ((basemem - boot_address) < bootMP_size)
+	if (((basemem * 1024) - boot_address) < bootMP_size)
 		boot_address -= PAGE_SIZE;	/* not enough, lower by 4k */
 	/* 3 levels of page table pages */
 	mptramp_pagetables = boot_address - (PAGE_SIZE * 3);