diff --git a/gemma/gm/text/_sampler.py b/gemma/gm/text/_sampler.py
index 944671a3..df14eecc 100644
--- a/gemma/gm/text/_sampler.py
+++ b/gemma/gm/text/_sampler.py
@@ -572,7 +572,7 @@ def _normalize_token(tokenizer, token: str | int) -> int:
   token_id = tokenizer.encode(token)
   if len(token_id) != 1:
     raise ValueError(
-        'Invalid token: {token!r}. `stop_token`s and `forbidden_token`s must'
+        f'Invalid token: {token!r}. `stop_token`s and `forbidden_token`s must'
         ' map to single token ids in the vocab.'
     )
   (token_id,) = token_id
diff --git a/gemma/research/t5gemma/sampling.py b/gemma/research/t5gemma/sampling.py
index 84a16ac4..85415f36 100644
--- a/gemma/research/t5gemma/sampling.py
+++ b/gemma/research/t5gemma/sampling.py
@@ -510,7 +510,7 @@ def _normalize_token(tokenizer, token: str | int) -> int:
   token_id = tokenizer.encode(token)
   if len(token_id) != 1:
     raise ValueError(
-        'Invalid forbidden token: {token!r}. Forbidden tokens must map to'
+        f'Invalid forbidden token: {token!r}. Forbidden tokens must map to'
         ' single token ids in the vocab.'
     )
   (token_id,) = token_id