diff --git a/src/mame/includes/btime.h b/src/mame/includes/btime.h
index 0d26d875a44..8856bcacf80 100644
--- a/src/mame/includes/btime.h
+++ b/src/mame/includes/btime.h
@@ -56,7 +56,7 @@ public:
 
 	/* devices */
 	required_device<cpu_device> m_maincpu;
-	required_device<cpu_device> m_audiocpu;
+	optional_device<cpu_device> m_audiocpu;
 
 	DECLARE_WRITE8_MEMBER(audio_nmi_enable_w);
 	DECLARE_WRITE8_MEMBER(lnc_w);
diff --git a/src/mame/includes/marineb.h b/src/mame/includes/marineb.h
index 1160da72b9d..26907203bea 100644
--- a/src/mame/includes/marineb.h
+++ b/src/mame/includes/marineb.h
@@ -24,7 +24,7 @@ public:
 
 	/* devices */
 	required_device<cpu_device> m_maincpu;
-	required_device<cpu_device> m_audiocpu;
+	optional_device<cpu_device> m_audiocpu;
 
 	UINT8     m_irq_mask;
 	DECLARE_WRITE8_MEMBER(irq_mask_w);