diff --git a/llms/mlx_lm/models/cache.py b/llms/mlx_lm/models/cache.py
index 49e3a198..8d5101d2 100644
--- a/llms/mlx_lm/models/cache.py
+++ b/llms/mlx_lm/models/cache.py
@@ -32,23 +32,21 @@ def make_prompt_cache(model: nn.Module, max_kv_size: Optional[int] = None) -> Li
         return [KVCache() for _ in range(num_layers)]
 
 
-def save_prompt_cache(
-    file_name: str, cache: List[Any], metadata: Optional[Dict[str, str]] = None
-):
+def save_prompt_cache(file_name: str, cache: List[Any], metadata: Dict[str, str] = {}):
     """
     Save a pre-computed prompt cache to a file.
 
     Args:
         file_name (str): The ``.safetensors`` file name.
         cache (List[Any]): The model state.
-        metadata (Optional[Dict[str, str]]): Optional metadata to save along
-            with model state..
+        metadata (Dict[str, str]): Optional metadata to save along with model
+            state.
     """
     cache_data = [c.state for c in cache]
-    cache_info = [c.meta_state if hasattr(c, "meta_state") else "" for c in cache]
+    cache_info = [c.meta_state for c in cache]
     cache_data = dict(tree_flatten(cache_data))
     cache_classes = [type(c).__name__ for c in cache]
-    cache_metadata = [cache_classes, cache_info, metadata or ""]
+    cache_metadata = [cache_classes, cache_info, metadata]
     cache_metadata = dict(tree_flatten(cache_metadata))
     mx.save_safetensors(file_name, cache_data, cache_metadata)
 
@@ -73,14 +71,33 @@ def load_prompt_cache(file_name, return_metadata=False):
     cache = [globals()[c]() for c in classes]
     for c, state, meta_state in zip(cache, arrays, info):
         c.state = state
-        if hasattr(c, "meta_state"):
-            c.meta_state = meta_state
+        c.meta_state = meta_state
     if return_metadata:
         return cache, metadata
     return cache
 
 
-class KVCache:
+class _BaseCache:
+    @property
+    def state(self):
+        return []
+
+    @state.setter
+    def state(self, v):
+        if v is not None and v:
+            raise ValueError("This cache has no state but a state was set.")
+
+    @property
+    def meta_state(self):
+        return {}
+
+    @state.setter
+    def meta_state(self, v):
+        if v is not None and v:
+            raise ValueError("This cache has no meta_state but a meta_state was set.")
+
+
+class KVCache(_BaseCache):
 
     def __init__(self):
         self.keys = None
@@ -128,7 +145,7 @@ class KVCache:
         self.offset = self.keys.shape[2]
 
 
-class RotatingKVCache:
+class RotatingKVCache(_BaseCache):
 
     def __init__(self, max_size=None, keep=0, step=256):
         self.keep = keep
@@ -259,7 +276,7 @@ class RotatingKVCache:
         )
 
 
-class MambaCache:
+class MambaCache(_BaseCache):
     def __init__(self):
         self.cache = [None, None]