Error: undefined method 'calc_min' for #<Wx::UpdateUIEvent:0

My application has been running fine for weeks, just today got this
error twice:

Error: undefined method ‘calc_min’ for #Wx::UpdateUIEvent:0xb18f9c4

OS: WXMSW
Ruby Version: 1.8.6
Backtrace:
E:/work/iTest2/app/main.rb:180:in calc_min' E:/work/iTest2/app/main.rb:180:inlayout’
E:/work/iTest2/app/main.rb:180:in process_event' E:/work/iTest2/app/main.rb:180:inmain_loop’

The line 180 is
app.main_loop

Env: Win XP, wxRuby 1.9.8 mswin32

Regards,
Zhimin