Hello all, Maybe there is something obvious I can't see, but I have this behavior for 6.8.2 ghci: $ghci ttest1p.hs GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling Main ( ttest1p.hs, interpreted ) Ok, modules loaded: Main. *Main> encode' [1..100] Loading package array-0.1.0.0 ... linking ... done. Loading package bytestring-0.9.0.1 ... linking ... done. [1,0,2,0,3,0,4,0,5,0,6,0,7,0,8,0,9,0,10,0,11,0,12,0,13,0,... // deleted *Main> B.pack [0..100] "\NUL\SOH\STX\ETX\EOT\ENQ\ACK\a\b\t\n\v\f\r\SO\SI\DLE\DC1\DC2\DC3\DC4\NAK\SYN\ETB\CAN\EM\SUB\ESC\FS\GS\RS\US !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcd" *Main> B.pack $ encode' [1..100] "*** Exception: divide by zero where ttest1p.hs: import qualified Data.ByteString as B encode' [] = [] encode' (x:xs) = if x==0 then 0:0:encode' xs else (x `mod` 256) : (x `div` 256) : encode' xs What is the difference, except list length and value structure? Where is my error? Thanks for any hint, Dusan