diff --git a/libsrc/c128/isfast.s b/libsrc/c128/isfast.s
index 01b597371..d40e2c7e9 100644
--- a/libsrc/c128/isfast.s
+++ b/libsrc/c128/isfast.s
@@ -14,6 +14,7 @@
 
         lda     VIC_CLK_128
         and     #$01
+        ldx     #$00
         rts
 
 .endproc