diff --git a/lib/unicode_normalize.rb b/lib/unicode_normalize.rb
index 3f359a0ff3..ef3f2ba6f1 100644
--- a/lib/unicode_normalize.rb
+++ b/lib/unicode_normalize.rb
@@ -7,18 +7,6 @@
 # additions to class String for Unicode normalization
 #++
 class String
-
-  # :call-seq:
-  #    str.unicode_normalize!(form=:nfc)
-  #
-  # Destructive version of String#unicode_normalize, doing Unicode
-  # normalization in place.
-  #
-  def unicode_normalize!(form = :nfc)
-    require 'unicode_normalize/normalize.rb'
-    unicode_normalize! form
-  end
-
   # :call-seq:
   #    str.unicode_normalized?(form=:nfc)
   #
diff --git a/lib/unicode_normalize/normalize.rb b/lib/unicode_normalize/normalize.rb
index 8bfbbfeea5..4302c6a975 100644
--- a/lib/unicode_normalize/normalize.rb
+++ b/lib/unicode_normalize/normalize.rb
@@ -160,10 +160,6 @@ module UnicodeNormalize  # :nodoc:
 end # module
 
 class String  # :nodoc:
-  def unicode_normalize!(form = :nfc)
-    replace(UnicodeNormalize.normalize(self, form))
-  end
-
   def unicode_normalized?(form = :nfc)
     UnicodeNormalize.normalized?(self, form)
   end
diff --git a/string.c b/string.c
index 6b8f2eaf6f..0d8b62780b 100644
--- a/string.c
+++ b/string.c
@@ -9615,7 +9615,6 @@ rb_str_unicode_normalize(int argc, VALUE *argv, VALUE str)
 	rb_require("unicode_normalize/normalize.rb");
 	UnicodeNormalizeRequired = 1;
     }
-    /* return rb_funcall2(str, id_unicode_normalize, argc, argv); */
     if (argc==0)
 	return rb_funcall(mUnicodeNormalize, id_normalize, 1, str);
     else if (argc==1)
@@ -9624,6 +9623,28 @@ rb_str_unicode_normalize(int argc, VALUE *argv, VALUE str)
 	rb_raise(rb_eArgError, "too many arguments to unicode_normalize");
 }
 
+/*
+ *  call-seq:
+ *    str.unicode_normalize!(form=:nfc)
+ *
+ *  Destructive version of String#unicode_normalize, doing Unicode
+ *  normalization in place.
+ */
+static VALUE
+rb_str_unicode_normalize_bang(int argc, VALUE *argv, VALUE str)
+{
+    if (!UnicodeNormalizeRequired) {
+	rb_require("unicode_normalize/normalize.rb");
+	UnicodeNormalizeRequired = 1;
+    }
+    if (argc==0)
+	return rb_str_replace(str, rb_funcall(mUnicodeNormalize, id_normalize, 1, str));
+    else if (argc==1)
+	return rb_str_replace(str, rb_funcall(mUnicodeNormalize, id_normalize, 2, str, argv[0]));
+    else
+	rb_raise(rb_eArgError, "too many arguments to unicode_normalize!");
+}
+
 /**********************************************************************
  * Document-class: Symbol
  *
@@ -10277,6 +10298,7 @@ Init_String(void)
     id_normalize               = rb_intern("normalize");
 
     rb_define_method(rb_cString, "unicode_normalize", rb_str_unicode_normalize, -1);
+    rb_define_method(rb_cString, "unicode_normalize!", rb_str_unicode_normalize_bang, -1);
 
     rb_fs = Qnil;
     rb_define_hooked_variable("$;", &rb_fs, 0, rb_fs_setter);