python - tokenizer.train_from_iterator throwing TypeError: expected string or buffer - Stack Overflow

I'm trying to build a custom Tokenizer for a project, and I'm getting error for tokenizer.train_from_iterator:

class IngenxPreTokenizer:
    def pre_tokenize(self, pretok: PreTokenizedString):
        processed = self.base_tokenizer.process_text(pretok)
        normalized_tokens = []
        current_offset = 0
        for token in processed:
            token_len = len(token)
            normalized_tokens.append((
                token,
                (current_offset, current_offset + token_len)
            ))
            current_offset += token_len + 1
            
        pretok.tokens = normalized_tokens
        return pretok

class IngenxTokenTrainer:
    def __init__(self,df,size_dataset =240340,vocab_size=150000,min_freq = 5,batch_size=1000):
        self.tokenizer = IngenxTokenizer()
        self.df = df
        self.size_dataset = size_dataset
        self.vocab_size = vocab_size
        self.min_freq = min_freq
        self.batch_size=1000
        self.special_tokens = ["<|unk|>","<|pad|>","</|eos|>",]
        self.training_corpus = self.preprare_dataset()
        
    def preprare_dataset(self):
        X2 = np.random.choice(len(self.df), size=self.size_dataset, replace=False)
        training_texts = [f"{df.iloc[i]['problem']} {df.iloc[i]['solution']}" for i in X2]
        return examples
        
    def get_training_corpus(self):
        dataset = self.training_corpus
        with tqdm(total=len(dataset), desc="Processing training corpus", unit="text") as pbar:
            for text in dataset:
                pbar.update(1)
                yield text
                
    def train_tokenizer(self):
        tokenizer = Tokenizer(BPE())
        tokenizer.pre_tokenizer = PreTokenizer.custom(IngenxPreTokenizer())

        trainer = BpeTrainer(
            vocab_size=self.vocab_size,
            min_frequency=self.min_freq,
            special_tokens=self.special_tokens
        )
        tokenizer.train_from_iterator(self.get_training_corpus(),trainer=trainer, length=len(self.training_corpus))
        tokenizer.save("ingenx_tokenizewr.json")
        return tokenizer

The error:

Exception Traceback (most recent call last) <ipython-input-47-3f931020c7fd> in <cell line: 1>() ----> 1 a.train_tokenizer() <ipython-input-44-3011da9bf75a> in train_tokenizer(self) 41 special_tokens=self.special_tokens 42 ) ---> 43 tokenizer.train_from_iterator(self.get_training_corpus(),trainer=trainer, length=len(self.training_corpus)) 44 tokenizer.save("ingenx_tokenizewr.json") 45 return tokenizer Exception: TypeError: expected string or buffer

I'm not able to figure out what am I doing wrong here. I also referred to the documentation and I followed every single step but I'm still getting the error.