diff --git a/libsrc/c64/get_ostype.s b/libsrc/c64/get_ostype.s
index b04c98576..e2db72ae2 100644
--- a/libsrc/c64/get_ostype.s
+++ b/libsrc/c64/get_ostype.s
@@ -14,6 +14,7 @@
 
 .proc	_get_ostype
 
+        ldx     #$00            ; Clear high byte
 	lda	$ff80
 	rts