TestBootstrapAfterUpgrade.test_upgrade_with_range_tombstone_eoc_0 upgrade test is failing with TypeError

self = <upgrade_tests.storage_engine_upgrade_test.TestBootstrapAfterUpgrade object at 0x7f8db9908240>

    @since('3.0', max_version='3.99')
    def test_upgrade_with_range_tombstone_eoc_0(self):
        """
            Check sstable upgrading when the sstable contains a range tombstone with EOC=0.
    
            @jira_ticket CASSANDRA-12423
            """
        session = self._setup_cluster(cluster_options={'start_rpc': 'true'})
    
        session.execute("CREATE TABLE rt (id INT, c1 TEXT, c2 TEXT, v INT, PRIMARY KEY (id, c1, c2)) "
                        "with compact storage and compression = {'sstable_compression': ''};")
    
        range_delete = {
            i32(1): {
                'rt': [Mutation(deletion=Deletion(2470761440040513,
                                                  predicate=SlicePredicate(slice_range=SliceRange(
>                                                     start=composite('a', eoc='\x00'),
                                                      finish=composite('asd', eoc='\x00')))))]
            }
        }

upgrade_tests/storage_engine_upgrade_test.py:434: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

item1 = b'a', item2 = None, eoc = '\x00'

    def composite(item1, item2=None, eoc=b'\x00'):
        if isinstance(item1, str):
            item1 = utf8encode(item1)
        if isinstance(item2, str):
            item2 = utf8encode(item2)
    
>       packed = _i16(len(item1)) + item1 + eoc
E       TypeError: can't concat str to bytes

thrift_test.py:153: TypeError