keras-team/keras · error · ValueError

Cannot set an empty vocabulary. Received: vocabulary={vocabu

Error message

Cannot set an empty vocabulary. Received: vocabulary={vocabulary}

What it means

An empty vocabulary would leave the lookup table with zero entries and break every downstream index computation, so set_vocabulary rejects arrays of size 0 instead of silently continuing.

Source

Thrown at keras/src/layers/preprocessing/index_lookup.py:493

                "when not executing eagerly. "
                "Create this layer or call `set_vocabulary()` "
                "outside of any traced function."
            )

        # TODO(mattdangerw): for better performance we should rewrite this
        # entire function to operate on tensors and convert vocabulary to a
        # tensor here.
        if tf.is_tensor(vocabulary):
            vocabulary = self._tensor_vocab_to_numpy(vocabulary)
        elif isinstance(vocabulary, (list, tuple)):
            vocabulary = np.array(vocabulary)
        if tf.is_tensor(idf_weights):
            idf_weights = idf_weights.numpy()
        elif isinstance(idf_weights, (list, tuple)):
            idf_weights = np.array(idf_weights)

        if vocabulary.size == 0:
            raise ValueError(
                "Cannot set an empty vocabulary. "
                f"Received: vocabulary={vocabulary}"
            )

        oov_start = self._oov_start_index()
        token_start = self._token_start_index()
        special_tokens = [self.mask_token] * oov_start + [
            self.oov_token
        ] * self.num_oov_indices
        found_special_tokens = np.array_equal(
            special_tokens, vocabulary[:token_start]
        )
        if found_special_tokens:
            tokens = vocabulary[token_start:]
        else:
            tokens = vocabulary

        repeated_tokens = self._find_repeated_tokens(tokens)

View on GitHub (pinned to 7a34a03db6)

Solutions

1. Check `len(vocab) > 0` before calling set_vocabulary.
2. If the vocabulary came from adapt, inspect the input dataset — it is yielding nothing.

Example fix

# before
layer.set_vocabulary(vocab)

# after
assert len(vocab) > 0, 'vocabulary is empty'
layer.set_vocabulary(vocab)

if len(vocab) == 0:
    raise ValueError('refusing to set an empty vocabulary')
layer.set_vocabulary(vocab)

def has_tokens(v) -> bool:
    return len(v) > 0

When it happens

Trigger: `set_vocabulary(np.array([]))` or an empty list; adapt() on a dataset slice that yields no tokens; parsing a vocabulary file to zero lines.

Common situations: Adapting on a fully filtered-out dataset; empty vocabulary files; conditional data paths that occasionally produce no rows.

Related errors

• Vocabulary file {vocabulary} does not exist.
• The passed vocabulary has at least one repeated term. Please
• Found reserved mask token at unexpected location in `vocabul
• Found reserved OOV token at unexpected location in `vocabula
• Attempted to set a vocabulary larger than the maximum vocab

AI-assisted analysis of keras-team/keras@7a34a03db6 (2026-08-25). Data as JSON: /api/errors/7341f8ac2282f974. Report an issue: GitHub.