iRule [string range...] not chunking data properly

In fact I have just grabbed this from an iRule i recently wrote. Almost the same, with fixed values. Note the end is 99 but the increment is 100. As you say, string range is zero-indexed

for {set start 0; set end 99} {$start < [HTTP::payload length]} {incr start 100;incr end 100} {
            log local0.debug "[string range [HTTP::payload] $start $end ]"
        }