diff --git a/hash.c b/hash.c
index 6ec20da95a..87438dfbf8 100644
--- a/hash.c
+++ b/hash.c
@@ -2374,18 +2374,25 @@ rb_hash_flatten(int argc, VALUE *argv, VALUE hash)
 {
     VALUE ary;
 
-    ary = rb_ary_new_capa(RHASH_SIZE(hash) * 2);
-    rb_hash_foreach(hash, flatten_i, ary);
     if (argc) {
 	int level = NUM2INT(*argv);
+	if (level == 0) return rb_hash_to_a(hash);
+
+	ary = rb_ary_new_capa(RHASH_SIZE(hash) * 2);
+	rb_hash_foreach(hash, flatten_i, ary);
 	if (level - 1 > 0) {
 	    *argv = INT2FIX(level - 1);
 	    rb_funcall2(ary, id_flatten_bang, argc, argv);
 	}
-	else if (level == -1) {
+	else if (level < 0) {
 	    rb_funcall2(ary, id_flatten_bang, 0, 0);
 	}
     }
+    else {
+	ary = rb_ary_new_capa(RHASH_SIZE(hash) * 2);
+	rb_hash_foreach(hash, flatten_i, ary);
+    }
+
     return ary;
 }
 
diff --git a/test/ruby/test_hash.rb b/test/ruby/test_hash.rb
index e2cafb620c..c0745dd166 100644
--- a/test/ruby/test_hash.rb
+++ b/test/ruby/test_hash.rb
@@ -974,7 +974,7 @@ class TestHash < Test::Unit::TestCase
 
     a =  @cls[1=> "one", 2 => [2,"two"], 3 => [3, ["three"]]]
     assert_equal([1, "one", 2, [2, "two"], 3, [3, ["three"]]], a.flatten)
-    assert_equal([1, "one", 2, [2, "two"], 3, [3, ["three"]]], a.flatten(0))
+    assert_equal([[1, "one"], [2, [2, "two"]], [3, [3, ["three"]]]], a.flatten(0))
     assert_equal([1, "one", 2, [2, "two"], 3, [3, ["three"]]], a.flatten(1))
     assert_equal([1, "one", 2, 2, "two", 3, 3, ["three"]], a.flatten(2))
     assert_equal([1, "one", 2, 2, "two", 3, 3, "three"], a.flatten(3))