diff --git a/bios/int15.c b/bios/int15.c
index e1a1ce4..faf5343 100644
--- a/bios/int15.c
+++ b/bios/int15.c
@@ -2,11 +2,17 @@
 
 #include "kvm/e820.h"
 
+#include <asm/processor-flags.h>
+
 bioscall void int15_handler(struct biosregs *regs)
 {
 	switch (regs->eax) {
 	case 0xe820:
 		e820_query_map(regs);
 		break;
+	default:
+		/* Set CF to indicate failure.  */
+		regs->eflags	|= X86_EFLAGS_CF;
+		break;
 	}
 }