Tkinter text entry with pyHook hangs GUI window

I have a Tkinter GUI application that I need to enter text in. I cannot assume that the application will have focus, so I implemented pyHook, keylogger-style.

When the GUI window does not have focus, text entry works just fine and the StringVar updates correctly. When the GUI window does have focus and I try to enter text, the whole thing crashes.

i.e., if I click on the console window or anything else after launching the program, text entry works. If I try entering text immediately (the GUI starts with focus), or I refocus the window at any point and enter text, it crashes.

What's going on?

Below is a minimal complete verifiable example to demonstrate what I mean:

from Tkinter import *
import threading
import time

try:
    import pythoncom, pyHook
except ImportError:
    print 'The pythoncom or pyHook modules are not installed.'

# main gui box
class TestingGUI:
    def __init__(self, root):

        self.root = root
        self.root.title('TestingGUI')

        self.search = StringVar()
        self.searchbox = Label(root, textvariable=self.search) 
        self.searchbox.grid()

    def ButtonPress(self, scancode, ascii):
        self.search.set(ascii)

root = Tk()
TestingGUI = TestingGUI(root)

def keypressed(event):
    key = chr(event.Ascii)
    threading.Thread(target=TestingGUI.ButtonPress, args=(event.ScanCode,key)).start()
    return True

def startlogger():
    obj = pyHook.HookManager()
    obj.KeyDown = keypressed
    obj.HookKeyboard()
    pythoncom.PumpMessages()

# need this to run at the same time
logger = threading.Thread(target=startlogger)
# quits on main program exit
logger.daemon = True
logger.start()

# main gui loop
root.mainloop()