nick completion in IRC throws exception

In a MUC room, seen in IRC, I get the exception below when I try to use nick autocompletion:
e.g. there is a nick like Amiga4000, and I enter "Hi A" and then use tab key, I get the exception below:

invalid command name ""
invalid command name ""
    while executing
"$wtext index "insert -1 c wordstart""
    (procedure "Complete" line 4)
    invoked from within
"Complete <email address hidden>"
    (in namespace inscope "::IRCActions" script line 1)
    invoked from within
"::namespace inscope ::IRCActions {Complete <email address hidden>}"
    (command bound to event)