[BUG] Readline.completion_append_character in latest snapshot

In ruby 1.9.0 (2004-11-18) [i686-linux], something changed in readline, 
which broke irb -r irb/completion.

$ irb -r irb/completion
/usr/local/lib/ruby/1.9/irb/completion.rb:187:in 
`completion_append_character=':TypeError: cannot convert nil into String

The offending line is:

Readline.completion_append_character = nil

but the problem is likely in readline.c, where the following changed:

@@ -195,7 +203,9 @@
      VALUE self, str;
  {
  #ifdef READLINE_21_OR_LATER
-    if (NIL_P(str) || !StringValuePtr(str) || !RSTRING(str)->len) {
+    rb_secure(4);
+    SafeStringValue(str);
+    if (NIL_P(str) || RSTRING(str)->len == 0) {
         rl_completion_append_character = '\0';
      } else {
         rl_completion_append_character = RSTRING(str)->ptr[0];



I guess SafeStringValue rejects Qnil.