diff --git a/libsrc/geos/disk/dio_cts.s b/libsrc/geos/disk/dio_cts.s
index f6118641e..c7871045d 100644
--- a/libsrc/geos/disk/dio_cts.s
+++ b/libsrc/geos/disk/dio_cts.s
@@ -69,7 +69,9 @@
 	cmp	#DRV_1581
 	beq	dio_cts1581
 
-; unknown device, return what you have got
+	lda	#DEV_NOT_FOUND	; unknown device
+	ldx	#0
+	beq	ret
 
 dio_ctsend:
 	ldy 	#1
diff --git a/libsrc/geos/disk/dio_stc.s b/libsrc/geos/disk/dio_stc.s
index ea6369fde..c64bb0802 100644
--- a/libsrc/geos/disk/dio_stc.s
+++ b/libsrc/geos/disk/dio_stc.s
@@ -66,7 +66,9 @@
 	cmp	#DRV_1581
 	beq	dio_stc1581
 
-;	...
+	lda	#DEV_NOT_FOUND	; unknown device
+	ldx	#0
+	beq	_ret
 
 dio_stcend:
 	ldy	#diopp_track