diff --git a/bootstraptest/test_ractor.rb b/bootstraptest/test_ractor.rb
index f008bc82a4add1..25af87b610ef83 100644
--- a/bootstraptest/test_ractor.rb
+++ b/bootstraptest/test_ractor.rb
@@ -1297,7 +1297,7 @@ class C
       s = 'str'
       trap(:INT){p s}
     }.join
-  rescue => Ractor::RemoteError
+  rescue Ractor::RemoteError
     a << :ok
   end
 }
diff --git a/lib/unicode_normalize/normalize.rb b/lib/unicode_normalize/normalize.rb
index 7872f8a0bcffdd..611f43ef2aa0ad 100644
--- a/lib/unicode_normalize/normalize.rb
+++ b/lib/unicode_normalize/normalize.rb
@@ -111,17 +111,22 @@ def self.nfc_one(string)
     start = nfd_string[0]
     last_class = CLASS_TABLE[start]-1
     accents = ''
+    result = ''
     nfd_string[1..-1].each_char do |accent|
       accent_class = CLASS_TABLE[accent]
       if last_class<accent_class and composite = COMPOSITION_TABLE[start+accent]
         start = composite
+      elsif accent_class == 0
+        result += start+accents
+        start = accent
+        accents = ''
+        last_class = -1
       else
         accents << accent
         last_class = accent_class
       end
     end
-    accents = nfc_one(accents) if accents.length>1 # TODO: change from recursion to loop
-    hangul_comp_one(start+accents)
+    hangul_comp_one(result+start+accents)
   end
 
   def self.normalize(string, form = :nfc)
diff --git a/test/test_unicode_normalize.rb b/test/test_unicode_normalize.rb
index 98f46551d33374..dd06d271310ad9 100644
--- a/test/test_unicode_normalize.rb
+++ b/test/test_unicode_normalize.rb
@@ -210,6 +210,15 @@ def test_us_ascii
     assert_equal true, ascii_string.unicode_normalized?(:nfkd)
   end
 
+  def test_bug_21559
+    str = "s\u{1611e}\u{323}\u{1611e}\u{307}\u{1611f}"
+    assert_equal str.unicode_normalize(:nfd), str.unicode_normalize(:nfc).unicode_normalize(:nfd)
+  end
+
+  def test_gurung_khema
+    assert_equal "\u{16121 16121 16121 16121 16121 1611E}", "\u{1611E 16121 16121 16121 16121 16121}".unicode_normalize
+  end
+
   def test_canonical_ordering
     a = "\u03B1\u0313\u0300\u0345"
     a_unordered1 = "\u03B1\u0345\u0313\u0300"