diff --git a/string.c b/string.c
index 49e3c18152..2fb67efe92 100644
--- a/string.c
+++ b/string.c
@@ -4158,7 +4158,7 @@ rb_str_sub_bang(int argc, VALUE *argv, VALUE str)
 	memcpy(p + beg0, rp, rlen);
 	len += rlen - plen;
 	STR_SET_LEN(str, len);
-	RSTRING_PTR(str)[len] = '\0';
+	TERM_FILL(&RSTRING_PTR(str)[len], TERM_LEN(str));
 	ENC_CODERANGE_SET(str, cr);
 	if (tainted) OBJ_TAINT(str);
 
diff --git a/test/-ext-/string/test_cstr.rb b/test/-ext-/string/test_cstr.rb
index a7cb0ebe9c..4535921836 100644
--- a/test/-ext-/string/test_cstr.rb
+++ b/test/-ext-/string/test_cstr.rb
@@ -60,6 +60,10 @@ class Test_StringCStr < Test::Unit::TestCase
     assert_wchars_term_char("a"*30) {|s| s[29,1] = ""}
   end
 
+  def test_wchar_sub!
+    assert_wchars_term_char("foobar") {|s| s.sub!(/#{"foo".encode(s.encoding)}/, "")}
+  end
+
   def assert_wchars_term_char(str)
     result = {}
     WCHARS.map do |enc|