WordLevel error: Missing [UNK] token from the vocabulary

Hi, I am trying to train a basic Word Level tokenizer based on a file data.txt containing

5174 5155 4749 4814 4832 4761 4523 4999 4860 4699 5024 4788 [UNK]

When I run my code

from tokenizers import Tokenizer
from tokenizers.models import WordLevel

tokenizer = Tokenizer(WordLevel(unk_token='[UNK]'))
tokenizer.train(files=['data.txt'])
tokenizer.encode('5155')

I get the error

Exception: WordLevel error: Missing [UNK] token from the vocabulary

Why is it still missing despite having [UNK] in data.txt and also setting unk_token='[UNK]'?

Any help is very appreciated!