diff --git a/ChangeLog b/ChangeLog
index d667367790..a5fe50de08 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sun Oct  4 15:11:48 2015  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* enc/euc_jp.c (mbc_case_fold): check given string is valid or not,
+	  and if invalid, return 1. [Bug #11486]
+
 Sun Oct  4 10:09:57 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* dir.c (rb_dir_getwd): normalize OS path to UTF-8 on OS X.
diff --git a/enc/euc_jp.c b/enc/euc_jp.c
index d0852c3daa..3088dfd64b 100644
--- a/enc/euc_jp.c
+++ b/enc/euc_jp.c
@@ -378,11 +378,12 @@ mbc_case_fold(OnigCaseFoldType flag,
     return 1;
   }
   else {
-    OnigCodePoint code;
     int len;
 
+    len = mbc_enc_len(p, end, enc);
+    if (len <= 0) return 1;
     code = get_lower_case(mbc_to_code(p, end, enc));
-    len = code_to_mbc(code, lower, enc);
+    /* assuming case folding in euc-jp doesn't change length */
     (*pp) += len;
     return len; /* return byte length of converted char to lower */
   }