Would it be because LEN(A$) already equals 0? So when you
FOR _I%=LEN(A$)-1 TO 0 STEP -1LEN(A$)-1 would be -1 and you are trying to increase it by -1 to get to 0 when you should increase by +1, unless I am misreading. EDIT: I think if the array was larger it might work. It's interesting to see the _I%=LEN rather than LEN being the condition to end the loop, so it is a little confusing to me.