diff --git a/flair/embeddings/transformer.py b/flair/embeddings/transformer.py
index a27b57ab9..fdb16eea2 100644
--- a/flair/embeddings/transformer.py
+++ b/flair/embeddings/transformer.py
@@ -278,11 +278,11 @@ def _reconstruct_word_ids_from_subtokens(embedding, tokens: list[str], subtokens
 
     special_tokens = []
     # check if special tokens exist to circumvent error message
-    if embedding.tokenizer._bos_token:
+    if embedding.tokenizer.bos_token is not None:
         special_tokens.append(embedding.tokenizer.bos_token)
-    if embedding.tokenizer._cls_token:
+    if embedding.tokenizer.cls_token is not None:
         special_tokens.append(embedding.tokenizer.cls_token)
-    if embedding.tokenizer._sep_token:
+    if embedding.tokenizer.sep_token is not None:
         special_tokens.append(embedding.tokenizer.sep_token)
 
     # iterate over subtokens and reconstruct tokens
@@ -1354,9 +1354,10 @@ def from_params(cls, params):
     def to_params(self):
         config_dict = self.model.config.to_dict()
 
-        # do not switch the attention implementation upon reload.
-        config_dict["attn_implementation"] = self.model.config._attn_implementation
-        config_dict.pop("_attn_implementation_autoset", None)
+        if hasattr(self.model.config, "_attn_implementation"):
+            # do not switch the attention implementation upon reload.
+            config_dict["attn_implementation"] = self.model.config._attn_implementation
+            config_dict.pop("_attn_implementation_autoset", None)
 
         super_params = super().to_params()
 
diff --git a/flair/models/tars_model.py b/flair/models/tars_model.py
index 4f5cb8573..f4171fdb2 100644
--- a/flair/models/tars_model.py
+++ b/flair/models/tars_model.py
@@ -383,7 +383,7 @@ def __init__(
 
         # transformer separator
         self.separator = str(self.tars_embeddings.tokenizer.sep_token)
-        if self.tars_embeddings.tokenizer._bos_token:
+        if self.tars_embeddings.tokenizer.bos_token is not None:
             self.separator += str(self.tars_embeddings.tokenizer.bos_token)
 
         self.prefix = prefix
@@ -718,9 +718,11 @@ def __init__(
         )
 
         # transformer separator
-        self.separator = str(self.tars_embeddings.tokenizer.sep_token)
-        if self.tars_embeddings.tokenizer._bos_token:
-            self.separator += str(self.tars_embeddings.tokenizer.bos_token)
+        self.separator = (
+            self.tars_embeddings.tokenizer.sep_token if self.tars_embeddings.tokenizer.sep_token is not None else ""
+        )
+        if self.tars_embeddings.tokenizer.bos_token is not None:
+            self.separator += self.tars_embeddings.tokenizer.bos_token
 
         self.prefix = prefix
         self.num_negative_labels_to_sample = num_negative_labels_to_sample
diff --git a/requirements.txt b/requirements.txt
index bb5ecafd4..2704114ac 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -20,6 +20,6 @@ tabulate>=0.8.10
 torch>=1.5.0,!=1.8
 tqdm>=4.63.0
 transformer-smaller-training-vocab>=0.2.3
-transformers[sentencepiece]>=4.18.0,<5.0.0
+transformers[sentencepiece]>=4.25.0,<5.0.0
 wikipedia-api>=0.5.7
 bioc<3.0.0,>=2.0.0