diff --git a/ext/dbm/dbm.c b/ext/dbm/dbm.c
index 1fdbaa0aca..03616fd4d8 100644
--- a/ext/dbm/dbm.c
+++ b/ext/dbm/dbm.c
@@ -259,8 +259,11 @@ fdbm_fetch(VALUE obj, VALUE keystr, VALUE ifnone)
     value = dbm_fetch(dbm, key);
     if (value.dptr == 0) {
       not_found:
-	if (ifnone == Qnil && rb_block_given_p())
-	    return rb_yield(rb_tainted_str_new(key.dptr, key.dsize));
+	if (NIL_P(ifnone) && rb_block_given_p()) {
+	    keystr = rb_str_dup(keystr);
+	    OBJ_TAINT(keystr);
+	    return rb_yield(keystr);
+	}
 	return ifnone;
     }
     return rb_tainted_str_new(value.dptr, value.dsize);
diff --git a/test/dbm/test_dbm.rb b/test/dbm/test_dbm.rb
index 9f7604be15..083a5c25ba 100644
--- a/test/dbm/test_dbm.rb
+++ b/test/dbm/test_dbm.rb
@@ -58,6 +58,14 @@ if defined? DBM
         assert_nil(@dbm_rdonly.delete("bar"))
       end
     end
+
+    def test_fetch_not_found
+      notfound = nil
+      result = Object.new
+      assert_same(result, @dbm_rdonly.fetch("bar") {|k| notfound = k; result})
+      assert_equal("bar", notfound)
+      assert_predicate(notfound, :tainted?)
+    end
   end
 
   class TestDBM < Test::Unit::TestCase