diff --git a/ChangeLog b/ChangeLog
index 67ddd26b2b..016be011cb 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Jan 23 13:39:48 2008  Tanaka Akira  <akr@fsij.org>
+
+	* re.c (rb_reg_prepare_re): fix SEGV by
+	  /a/ =~ "aa".force_encoding("utf-16be").
+
 Wed Jan 23 11:53:26 2008  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* string.c (str_mod_check, str_nth, str_offset): consitfied.
diff --git a/re.c b/re.c
index 67e56cdc61..46d0bbfcb7 100644
--- a/re.c
+++ b/re.c
@@ -955,7 +955,7 @@ rb_reg_prepare_re(VALUE re, VALUE str)
 
     rb_reg_check(re);
     /* ignorecase status */
-    if (rb_reg_fixed_encoding_p(re)) {
+    if (rb_reg_fixed_encoding_p(re) || !rb_enc_str_asciicompat_p(str)) {
         if (ENCODING_GET(re) != rb_enc_get_index(str) &&
             rb_enc_str_coderange(str) != ENC_CODERANGE_7BIT) {
             rb_raise(rb_eArgError,
@@ -994,6 +994,10 @@ rb_reg_prepare_re(VALUE re, VALUE str)
             RREGEXP(re)->str, RREGEXP(re)->str + RREGEXP(re)->len, enc,
             &fixed_enc, err);
 
+        if (unescaped == Qnil) {
+            rb_raise(rb_eArgError, "regexp preprocess failed: %s", err);
+        }
+
 	r = onig_new(&reg2, (UChar* )RSTRING_PTR(unescaped),
 		     (UChar* )(RSTRING_PTR(unescaped) + RSTRING_LEN(unescaped)),
 		     reg->options, enc,