With a Tcl compiled with <code>--enable-symbols</code> , the script


<code><verbatim>
package require Thread
set t1 [thread::create]
thread::send -async $t1 {lindex {}} [expr 0]


results in:


<code><verbatim>
alloc: invalid block


This happens because <code>threadSendFree</code>

obtains the length] of a <code>Tcl_Obj</code> before the string representation
for that object is generated, and a few lines later

string representation is generated].