diff --git a/src/mame/drivers/qix.c b/src/mame/drivers/qix.c
index e0a9d3884e2..9e9598cd84d 100644
--- a/src/mame/drivers/qix.c
+++ b/src/mame/drivers/qix.c
@@ -694,10 +694,12 @@ static MACHINE_CONFIG_DERIVED( slither, qix_base )
 	MCFG_DEVICE_MODIFY("pia1")
 	MCFG_PIA_READPA_HANDLER(READ8(qix_state, slither_trak_lr_r))
 	MCFG_PIA_WRITEPB_HANDLER(WRITE8(qix_state, slither_76489_0_w))
+	MCFG_PIA_READPB_HANDLER(NULL)
 
 	MCFG_DEVICE_MODIFY("pia2")
 	MCFG_PIA_READPA_HANDLER(READ8(qix_state, slither_trak_ud_r))
 	MCFG_PIA_WRITEPB_HANDLER(WRITE8(qix_state, slither_76489_1_w))
+	MCFG_PIA_READPB_HANDLER(NULL)
 
 	/* video hardware */
 	MCFG_FRAGMENT_ADD(slither_video)