diff --git a/ChangeLog b/ChangeLog
index 74c6a4d94c..13bea23e6b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Mon Mar 14 10:02:23 2016  Eric Wong  <e@80x24.org>
+
+	* ext/openssl/ossl_ssl.c (ossl_sslctx_setup): document as MT-unsafe
+	  [ruby-core:73803] [Bug #12069]
+
 Sun Mar 13 09:43:23 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* include/ruby/win32.h (O_SHARE_DELETE): change to fit Fixnum
diff --git a/ext/openssl/ossl_ssl.c b/ext/openssl/ossl_ssl.c
index 4075d6f001..90ce762a31 100644
--- a/ext/openssl/ossl_ssl.c
+++ b/ext/openssl/ossl_ssl.c
@@ -687,8 +687,8 @@ ossl_sslctx_set_options(VALUE self, VALUE options)
  *    ctx.setup => nil # thereafter
  *
  * This method is called automatically when a new SSLSocket is created.
- * Normally you do not need to call this method (unless you are writing an
- * extension in C).
+ * However, it is not thread-safe and must be called before creating
+ * SSLSocket objects in a multi-threaded program.
  */
 static VALUE
 ossl_sslctx_setup(VALUE self)