diff --git a/dspy/clients/__init__.py b/dspy/clients/__init__.py
index 43a5c9403a..7972bba8a4 100644
--- a/dspy/clients/__init__.py
+++ b/dspy/clients/__init__.py
@@ -14,12 +14,14 @@
 
 DISK_CACHE_DIR = os.environ.get("DSPY_CACHEDIR") or os.path.join(Path.home(), ".dspy_cache")
 DISK_CACHE_LIMIT = int(os.environ.get("DSPY_CACHE_LIMIT", 3e10))  # 30 GB default
+
+
 def configure_cache(
     enable_disk_cache: bool | None = True,
     enable_memory_cache: bool | None = True,
     disk_cache_dir: str | None = DISK_CACHE_DIR,
     disk_size_limit_bytes: int | None = DISK_CACHE_LIMIT,
-    memory_max_entries: int | None = 1000000,
+    memory_max_entries: int = 1000000,
 ):
     """Configure the cache for DSPy.
 
@@ -28,7 +30,8 @@ def configure_cache(
         enable_memory_cache: Whether to enable in-memory cache.
         disk_cache_dir: The directory to store the on-disk cache.
         disk_size_limit_bytes: The size limit of the on-disk cache.
-        memory_max_entries: The maximum number of entries in the in-memory cache.
+        memory_max_entries: The maximum number of entries in the in-memory cache. To allow the cache to grow without
+                            bounds, set this parameter to `math.inf` or a similar value.
     """
 
     DSPY_CACHE = Cache(
@@ -40,6 +43,7 @@ def configure_cache(
     )
 
     import dspy
+
     # Update the reference to point to the new cache
     dspy.cache = DSPY_CACHE
 
@@ -47,6 +51,7 @@ def configure_cache(
 litellm.telemetry = False
 litellm.cache = None  # By default we disable LiteLLM cache and use DSPy on-disk cache.
 
+
 def _get_dspy_cache():
     disk_cache_dir = os.environ.get("DSPY_CACHEDIR") or os.path.join(Path.home(), ".dspy_cache")
     disk_cache_limit = int(os.environ.get("DSPY_CACHE_LIMIT", 3e10))
@@ -71,6 +76,7 @@ def _get_dspy_cache():
         )
     return _dspy_cache
 
+
 DSPY_CACHE = _get_dspy_cache()
 
 if "LITELLM_LOCAL_MODEL_COST_MAP" not in os.environ:
diff --git a/dspy/clients/cache.py b/dspy/clients/cache.py
index 4773306ccc..a46f27723e 100644
--- a/dspy/clients/cache.py
+++ b/dspy/clients/cache.py
@@ -29,7 +29,7 @@ def __init__(
         enable_memory_cache: bool,
         disk_cache_dir: str,
         disk_size_limit_bytes: int | None = 1024 * 1024 * 10,
-        memory_max_entries: int | None = 1000000,
+        memory_max_entries: int = 1000000,
     ):
         """
         Args:
@@ -43,6 +43,10 @@ def __init__(
         self.enable_disk_cache = enable_disk_cache
         self.enable_memory_cache = enable_memory_cache
         if self.enable_memory_cache:
+            if memory_max_entries is None:
+                raise ValueError("`memory_max_entries` cannot be None. Use `math.inf` if you need an unbounded cache.")
+            elif memory_max_entries <= 0:
+                raise ValueError(f"`memory_max_entries` must be a positive number, but received {memory_max_entries}")
             self.memory_cache = LRUCache(maxsize=memory_max_entries)
         else:
             self.memory_cache = {}
diff --git a/tests/clients/test_cache.py b/tests/clients/test_cache.py
index 20e0af3232..02fb27f01a 100644
--- a/tests/clients/test_cache.py
+++ b/tests/clients/test_cache.py
@@ -71,6 +71,27 @@ def test_initialization(tmp_path):
     assert disabled_cache.disk_cache == {}
 
 
+def test_invalid_cache_initialization():
+    with pytest.raises(ValueError, match=r"`memory_max_entries` must be a positive number, but received -1"):
+        Cache(
+            enable_disk_cache=False,
+            enable_memory_cache=True,
+            disk_cache_dir="",
+            disk_size_limit_bytes=0,
+            memory_max_entries=-1,
+        )
+    with pytest.raises(
+        ValueError, match=r"`memory_max_entries` cannot be None. Use `math.inf` if you need an unbounded cache."
+    ):
+        Cache(
+            enable_disk_cache=False,
+            enable_memory_cache=True,
+            disk_cache_dir="",
+            disk_size_limit_bytes=0,
+            memory_max_entries=None,
+        )
+
+
 def test_cache_key_generation(cache):
     """Test cache key generation with different types of inputs."""
     # Test with simple dictionary