diff --git a/nanochat/checkpoint_manager.py b/nanochat/checkpoint_manager.py
index c008ec2..d1e0a07 100644
--- a/nanochat/checkpoint_manager.py
+++ b/nanochat/checkpoint_manager.py
@@ -111,7 +111,7 @@ def build_model(checkpoint_dir, step, device, phase):
     # Load the Tokenizer
     tokenizer = get_tokenizer()
     # Sanity check: compatibility between model and tokenizer
-    assert tokenizer.get_vocab_size() == model_config_kwargs["vocab_size"]
+    assert tokenizer.get_vocab_size() == model_config_kwargs["vocab_size"], f"Tokenizer vocab size {tokenizer.get_vocab_size()} does not match model config vocab size {model_config_kwargs['vocab_size']}"
     return model, tokenizer, meta_data