diff --git a/textgrad/engine_experimental/base.py b/textgrad/engine_experimental/base.py
index e50e613..8381446 100644
--- a/textgrad/engine_experimental/base.py
+++ b/textgrad/engine_experimental/base.py
@@ -38,7 +38,7 @@ class EngineLM(ABC):
     def __init__(self, model_string: str,
                  system_prompt: str = "You are a helpful, creative, and smart assistant.",
                  is_multimodal: bool = False,
-                 cache=Union[dc.Cache, bool]):
+                 cache: Union[dc.Cache, bool] = False):
 
         """
         Base class for the engines.
diff --git a/textgrad/engine_experimental/openai.py b/textgrad/engine_experimental/openai.py
index 2fea80f..a73d40a 100644
--- a/textgrad/engine_experimental/openai.py
+++ b/textgrad/engine_experimental/openai.py
@@ -20,7 +20,7 @@ class OpenAIEngine(EngineLM):
     def __init__(self, model_string: str,
                  system_prompt: str = DEFAULT_SYSTEM_PROMPT,
                  is_multimodal: bool = False,
-                 cache=Union[dc.Cache, bool]):
+                 cache: Union[dc.Cache, bool] = False):
 
         self.validate()
 
@@ -92,7 +92,7 @@ def __init__(self,
                  model_string: str,
                  system_prompt: str = DEFAULT_SYSTEM_PROMPT,
                  is_multimodal: bool = False,
-                 cache=Union[dc.Cache, bool]):
+                 cache: Union[dc.Cache, bool] = False):
 
             self.client = client