diff --git a/ChangeLog b/ChangeLog
index 8c7bd61164..4690f79c0e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sat Nov 19 14:06:07 2016  CHIKANAGA Tomoyuki  <nagachika@ruby-lang.org>
+
+	* iseq.c (proc_dup): don't duplicate sym_procs.  [Fix GH-1479]
+	  [ruby-core:78100] [Bug #12927]
+	  Based on the patch provided by Emiliano Ritiro.
+
 Sat Nov 19 11:48:47 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* iseq.c (iseqw_s_compile_file): deal with syntax error as well as
diff --git a/proc.c b/proc.c
index 9df55c45c4..755b03ea30 100644
--- a/proc.c
+++ b/proc.c
@@ -65,11 +65,13 @@ typedef struct {
     VALUE env[3]; /* me, specval, envval */
 } cfunc_proc_t;
 
+#define IS_SYM_PROC(proc) (proc->block.ep == ((const cfunc_proc_t *)proc)->env+1)
+
 static size_t
 proc_memsize(const void *ptr)
 {
     const rb_proc_t *proc = ptr;
-    if (proc->block.ep == ((const cfunc_proc_t *)ptr)->env+1)
+    if (IS_SYM_PROC(proc))
 	return sizeof(cfunc_proc_t);
     return sizeof(rb_proc_t);
 }
@@ -111,6 +113,8 @@ proc_dup(VALUE self)
     rb_proc_t *dst;
 
     GetProcPtr(self, src);
+    if (IS_SYM_PROC(src))
+	return self;
     procval = rb_proc_alloc(rb_cProc);
     GetProcPtr(procval, dst);
     *dst = *src;
diff --git a/version.h b/version.h
index d7af154d2a..cbf5191573 100644
--- a/version.h
+++ b/version.h
@@ -1,6 +1,6 @@
 #define RUBY_VERSION "2.3.3"
 #define RUBY_RELEASE_DATE "2016-11-19"
-#define RUBY_PATCHLEVEL 220
+#define RUBY_PATCHLEVEL 221
 
 #define RUBY_RELEASE_YEAR 2016
 #define RUBY_RELEASE_MONTH 11