diff --git a/Lib/site.py b/Lib/site.py
--- a/Lib/site.py
+++ b/Lib/site.py
@@ -383,18 +383,26 @@ def enablerlcompleter():
         import atexit
         try:
             import readline
             import rlcompleter
         except ImportError:
             return
 
         # Reading the initialization (config) file may not be enough to set a
-        # completion key, so we set one first and then read the file
-        if 'libedit' in getattr(readline, '__doc__', ''):
+        # completion key, so we set one first and then read the file.
+        #
+        # On MacOS X the readline module can be implemented using the libedit
+        # library instead of GNU readline. See msg123703 in issue 5845 for more
+        # information.
+        #
+        # On Windows the pyreadline module's __doc__ attribute is None.
+        # See issue 18852.
+        readline_doc = getattr(readline, '__doc__', '')
+        if readline_doc is not None and 'libedit' in readline_doc:
             readline.parse_and_bind('bind ^I rl_complete')
         else:
             readline.parse_and_bind('tab: complete')
 
         try:
             readline.read_init_file()
         except OSError:
             # An OSError here could have many causes, but the most likely one