Bug? or my error?

[ttw]

I have been spending some time trying to get the follow program (and several similar programs) to work. They (may) work for a while then failW with the error "Error: invalid input" with the "info" button showing the lines with "nextprime" and "append" showing. Sometimes the programs work for a while.

I think I must have something global being changed, but I cannot find it.

EXPORT Erro1(NN)
BEGIN
LOCAL CC,II,LL;
cc:=2;LL:={2};
FOR II FROM 2 TO NN DO
CC:=nextprime(CC);
LL:=append(LL,CC);
END;
PRINT (LL);
END;

I have tried changing variable names to single letter (global) or making temporaries. When I insert PRINT statements before the offending lines, these just print the number 2.