diff --git a/ChangeLog b/ChangeLog
index 0f2b0706c5..f07be877e6 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Tue Mar 29 01:41:10 2016  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (enc_succ_alnum_char): try to skip an invalid character
+	  gap between GREEK CAPITAL RHO and SIGMA.
+	  [ruby-core:74478] [Bug #12204]
+
 Tue Mar 29 01:27:52 2016  Victor Nawothnig  <Victor.Nawothnig@gmail.com>
 
 	* parse.y (parse_numvar): NTH_REF must be less than a half of
diff --git a/string.c b/string.c
index efae3d607f..3edc3f55c3 100644
--- a/string.c
+++ b/string.c
@@ -3241,6 +3241,10 @@ enc_succ_alnum_char(char *p, long len, rb_encoding *enc, char *carry)
     int range;
     char save[ONIGENC_CODE_TO_MBC_MAXLEN];
 
+    /* skip 03A2, invalid char between GREEK CAPITAL LETTERS */
+    int try;
+    const int max_gaps = 1;
+
     c = rb_enc_mbc_to_codepoint(p, p+len, enc);
     if (rb_enc_isctype(c, ONIGENC_CTYPE_DIGIT, enc))
         ctype = ONIGENC_CTYPE_DIGIT;
@@ -3250,11 +3254,13 @@ enc_succ_alnum_char(char *p, long len, rb_encoding *enc, char *carry)
         return NEIGHBOR_NOT_CHAR;
 
     MEMCPY(save, p, char, len);
-    ret = enc_succ_char(p, len, enc);
-    if (ret == NEIGHBOR_FOUND) {
-        c = rb_enc_mbc_to_codepoint(p, p+len, enc);
-        if (rb_enc_isctype(c, ctype, enc))
-            return NEIGHBOR_FOUND;
+    for (try = 0; try <= max_gaps; ++try) {
+	ret = enc_succ_char(p, len, enc);
+	if (ret == NEIGHBOR_FOUND) {
+	    c = rb_enc_mbc_to_codepoint(p, p+len, enc);
+	    if (rb_enc_isctype(c, ctype, enc))
+		return NEIGHBOR_FOUND;
+	}
     }
     MEMCPY(p, save, char, len);
     range = 1;
diff --git a/test/ruby/test_m17n.rb b/test/ruby/test_m17n.rb
index f5c82dcde9..a4a2630edd 100644
--- a/test/ruby/test_m17n.rb
+++ b/test/ruby/test_m17n.rb
@@ -1632,4 +1632,9 @@ class TestM17N < Test::Unit::TestCase
     assert_match(escape_plain, 0x5b.chr(::Encoding::UTF_8), bug10670)
     assert_match(escape_plain, 0x5b.chr, bug10670)
   end
+
+  def test_greek_capital_gap
+    bug12204 = '[ruby-core:74478] [Bug #12204] GREEK CAPITAL RHO and SIGMA'
+    assert_equal("\u03A3", "\u03A1".succ, bug12204)
+  end
 end
diff --git a/version.h b/version.h
index 48ccf3032b..8a3e04b9c6 100644
--- a/version.h
+++ b/version.h
@@ -1,6 +1,6 @@
 #define RUBY_VERSION "2.2.5"
 #define RUBY_RELEASE_DATE "2016-03-29"
-#define RUBY_PATCHLEVEL 278
+#define RUBY_PATCHLEVEL 279
 
 #define RUBY_RELEASE_YEAR 2016
 #define RUBY_RELEASE_MONTH 3