diff -r 587f64d9ecf2 Lib/idlelib/PyShell.py
--- a/Lib/idlelib/PyShell.py	Tue Mar 06 17:54:06 2012 +0100
+++ b/Lib/idlelib/PyShell.py	Tue Mar 06 19:49:36 2012 +0100
@@ -1221,6 +1221,11 @@
         self.set_line_and_column()
 
     def write(self, s, tags=()):
+        if isinstance(s, str) and max(s) > '\uffff':
+            # Tk doesn't support outputting non-BMP characters
+            char = max(s)
+            start = s.find(char)
+            raise UnicodeEncodeError("UCS-2", char, start, start+1, 'Non-BMP character not supported in Tk')
         try:
             self.text.mark_gravity("iomark", "right")
             OutputWindow.write(self, s, tags, "iomark")