diff --git a/DSS/drivers/Input/MOUSE.ASM b/DSS/drivers/Input/MOUSE.ASM
index cb915b2..8c14443 100644
--- a/DSS/drivers/Input/MOUSE.ASM
+++ b/DSS/drivers/Input/MOUSE.ASM
@@ -1008,7 +1008,7 @@ M_INT:	IN	A,(SLOT3)
 	;!TEST SIO INT
 	IF	MOUSE_INT_ENABLED
 	 LD	A,%0011'1000		; return from int
-	 OUT	(Z84.SIO.Ch_B.Ctrl),A
+	 OUT	(Z84.SIO.Ch_A.Ctrl),A	; RFI is CH-A ONLY (per Z80 SIO spec)
 	ENDIF
 	RET