diff --git a/set.c b/set.c
index 221b9a07e1..0f72a8ea4d 100644
--- a/set.c
+++ b/set.c
@@ -1120,14 +1120,10 @@ set_merge_enum_into(VALUE set, VALUE arg)
         set_iter(arg, set_merge_i, (st_data_t)&args);
     }
     else if (RB_TYPE_P(arg, T_ARRAY)) {
-        long len = RARRAY_LEN(arg);
-        if (RARRAY_LEN(arg) != 0) {
-            set_table *into = RSET_TABLE(set);
-            RARRAY_PTR_USE(arg, ptr, {
-                for(; len > 0; len--, ptr++) {
-                    set_table_insert_wb(into, set, *ptr, NULL);
-                }
-            });
+        long i;
+        set_table *into = RSET_TABLE(set);
+        for (i=0; i<RARRAY_LEN(arg); i++) {
+            set_table_insert_wb(into, set, RARRAY_AREF(arg, i), NULL);
         }
     }
     else {
diff --git a/test/ruby/test_set.rb b/test/ruby/test_set.rb
index 2de6cdaaee..225b7da78c 100644
--- a/test/ruby/test_set.rb
+++ b/test/ruby/test_set.rb
@@ -632,6 +632,17 @@ class TC_Set < Test::Unit::TestCase
     }
   end
 
+  def test_merge_mutating_hash_bug_21305
+    a = (1..100).to_a
+    o = Object.new
+    o.define_singleton_method(:hash) do
+      a.clear
+      0
+    end
+    a.unshift o
+    assert_equal([o], Set.new.merge(a).to_a)
+  end
+
   def test_subtract
     set = Set[1,2,3]