diff --git a/string.c b/string.c
index e71263efef..abaae1e726 100644
--- a/string.c
+++ b/string.c
@@ -8019,6 +8019,7 @@ rb_str_codepoints(VALUE str)
 static VALUE
 rb_str_enumerate_grapheme_clusters(VALUE str, VALUE ary)
 {
+    VALUE orig = str;
     regex_t *reg_grapheme_cluster = NULL;
     static regex_t *reg_grapheme_cluster_utf8 = NULL;
     int encidx = ENCODING_GET(str);
@@ -8046,6 +8047,7 @@ rb_str_enumerate_grapheme_clusters(VALUE str, VALUE ary)
 	}
     }
 
+    if (!ary) str = rb_str_new_frozen(str);
     ptr = RSTRING_PTR(str);
     end = RSTRING_END(str);
 
@@ -8060,10 +8062,11 @@ rb_str_enumerate_grapheme_clusters(VALUE str, VALUE ary)
 	ENUM_ELEM(ary, rb_enc_str_new(ptr, len, enc));
 	ptr += len;
     }
+    RB_GC_GUARD(str);
     if (ary)
 	return ary;
     else
-	return str;
+	return orig;
 }
 
 /*
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index f662a83c12..c7f4521e27 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -919,6 +919,8 @@ CODE
     assert_equal ["\u000A", "\u0308"], "\u{a 308}".each_grapheme_cluster.to_a
     assert_equal ["\u000D", "\u0308"], "\u{d 308}".each_grapheme_cluster.to_a
     assert_equal ["a", "b", "c"], "abc".b.each_grapheme_cluster.to_a
+    s = ("x"+"\u{10ABCD}"*250000)
+    assert_empty(s.each_grapheme_cluster {s.clear})
   end
 
   def test_grapheme_clusters