diff --git a/gguf-py/gguf/vocab.py b/gguf-py/gguf/vocab.py
index de3e5edb557d7..76924d8f29f5e 100644
--- a/gguf-py/gguf/vocab.py
+++ b/gguf-py/gguf/vocab.py
@@ -109,8 +109,10 @@ def _try_load_merges_txt(self, path: Path) -> bool:
         return True
 
     def _set_special_token(self, typ: str, tid: Any) -> None:
-        if not isinstance(tid, int) or tid < 0:
+        if not isinstance(tid, int):
             return
+        if tid < 0:
+            raise ValueError(f'invalid value for special token type {typ}: {tid}')
         if self.n_vocab is None or tid < self.n_vocab:
             if typ in self.special_token_ids:
                 return