diff --git a/hw/audio/ac97.c b/hw/audio/ac97.c
index ffd8607ba6..bfe1c2555e 100644
--- a/hw/audio/ac97.c
+++ b/hw/audio/ac97.c
@@ -174,7 +174,7 @@ enum {
     CAS      = 0x34
 };
 
-#define GET_BM(index) (((index) >> 4) & 3)
+#define GET_BM(index) (((index) >> 4) & 7)
 
 static void po_callback (void *opaque, int free);
 static void pi_callback (void *opaque, int avail);