I seem have hit the same (or related) bug that Laurent Orseau submitted as bug 11017 (but in a completely different context that might help track it down) and I haven't been able to find a workaround. The actual error message I get is: string<?: expects type <string> as 1st argument, given: #f; other arguments were: "c29a751537a750b825f72c9ee37eea737a0f3a8b9cd41e2f58a86ac8158ca605f292b20a02896479" coming from somewhere in the bowels of DrRacket. I don't get the error when using Racket (i.e., MzScheme). I'm not sure if this machine was running the stock V5.0 or a post-V5.0 nightly build that fixed a PLaneT problem I was having. But, the pre-release 5.0.1 build gives the same error. I reloaded V4.2.5 and the code runs fine there. I was hoping that the string in the error message itself might give someone in the know some idea of the problem. Unfortunately, this happens in a rather large autogenerated agent-based simulation that isn't easily posted - although I can send the code and the development packages it uses to someone if they need it. A short extract of the 'main' program that runs: #lang scheme ;;; Agent Generation from Templates - PRIME (AGenT') by SET Corporation (an SAIC Company) ;;; Autogenerated PLT Scheme source code invest-in-pa-yangi-yangi-tucker---plausible-outcomes-212-main.ss ;;; Generated Monday, July 19th, 2010 12:33:42pm (require (planet williams/madness/madness) "invest-in-pa-yangi-yangi-tucker---plausible-outcomes-212-arguments.ss" "invest-in-pa-yangi-yangi-tucker---plausible-outcomes-212-argument-values.ss" "invest-in-pa-yangi-yangi-tucker---plausible-outcomes-212-data-collection.ss") ... <about 1300 SLOC redacted> ;;; ;;; Monte Carlo loop INVEST-IN-PA-YANGI-YANGI-TUCKER---PLAUSIBLE-OUTCOMES ;;; (define (INVEST-IN-PA-YANGI-YANGI-TUCKER---PLAUSIBLE-OUTCOMES n) (with-new-simulation-environment (let ((aggregate-instance-index (make-hasheq))) (for ((i (in-range n))) (INVEST-IN-PA-YANGI-YANGI-TUCKER i aggregate-instance-index)) (print-statistics aggregate-instance-index) (generate-exhibit-files aggregate-instance-index) (generate-results-XML aggregate-instance-index n)))) ;;; ;;; Execute Monte Carlo ;;; (INVEST-IN-PA-YANGI-YANGI-TUCKER---PLAUSIBLE-OUTCOMES 10) ;;; ;;; Module Contracts ;;; (provide (all-defined-out)) If I simply remove the call to (INVEST-IN-PA-YANGI-YANGI-TUCKER---PLAUSIBLE-OUTCOMES 10) and move it to it's own package: #lang scheme ;;; Agent Generation from Templates - PRIME (AGenT') by SET Corporation (an SAIC Company) ;;; Autogenerated PLT Scheme source code invest-in-pa-yangi-yangi-tucker---plausible-outcomes-212-monte-carlo.ss ;;; Generated Monday, July 19th, 2010 12:33:43pm (require "invest-in-pa-yangi-yangi-tucker---plausible-outcomes-212-main.ss") ;;; ;;; Execute Monte Carlo ;;; (INVEST-IN-PA-YANGI-YANGI-TUCKER---PLAUSIBLE-OUTCOMES 10) I get the error above. [Again, it runs under Racket (i.e., MzScheme).] Please let me know if there are some switches I can set or otherwise provide additional information. Ah, I just ran it with "No debugging or profiling" checked and got some additional information - seems interesting that I got more information with no debugging. But anyway the error printed is now ..\..\..\..\..\..\..\..\..\Program Files\Racket\collects\racket\private\sort.rkt:85:19: string<?: expects type <string> as 1st argument, given: #f; other arguments were: "146047b7b2121bf8f7b15d7c2ef396b710f394474c12d02f2de08868342b8264c39b7fe111a38bf5" It also gave a backtrace that I copied and pasted into the attached file. [Was a better way to do that?] Doug