Thanks for your help. Here's the output:
(ltk:ltktest)
package require Tk senddatastring [tk_messageBox -message {Read or write operation to stream #<output stream ""c:/Tcl/bin/wish85.exe" "- name" "LTK""> signaled an error. Explanation: Invalid argument.
Do you wish to invoke the debugger? } -title {Error} -type yesno -icon question] Read or write operation to stream #<output stream ""c:/Tcl/bin/wish85.exe" "-name" "LTK""> signaled an error. Explanation: Invalid argument. Broken at SI:BYTECODES.No restarts available. Broken at LTK:MESSAGE-BOX. File: #P"C:/lisp/lib/ltk-0.91/ltk.lisp" (Form #398)
:b
Backtrace: LTK:MESSAGE-BOX > ltk::debug-popup > ltk::debug-error > ltk:send-wish > ltk::init-wish > lambda > lambda > lt k::use-default-debugger > lambda > ltk:start-wish > ltk:start-wish > ltk:call-with-ltk > ltk:ltktest > si:bytecodes > si :bytecodes > si:bytecodes