diff --git a/litellm/proxy/proxy_server.py b/litellm/proxy/proxy_server.py
index 927d0ae8e8..2672ec252d 100644
--- a/litellm/proxy/proxy_server.py
+++ b/litellm/proxy/proxy_server.py
@@ -2105,13 +2105,19 @@ async def _init_and_increment_window_spend_counter(
     window_start: Optional[datetime],
     increment: float,
 ):
-    if window_start is not None:
-        await _ensure_window_spend_counter_initialized(
-            counter_key=counter_key,
-            entity_type=entity_type,
-            entity_id=entity_id,
-            window_start=window_start,
+    if window_start is None:
+        verbose_proxy_logger.warning(
+            "Skipping spend counter increment for invalid budget window %s",
+            counter_key,
         )
+        return
+
+    await _ensure_window_spend_counter_initialized(
+        counter_key=counter_key,
+        entity_type=entity_type,
+        entity_id=entity_id,
+        window_start=window_start,
+    )
     await _increment_spend_counter_cache(counter_key=counter_key, increment=increment)
 
 
diff --git a/tests/test_litellm/proxy/test_proxy_server.py b/tests/test_litellm/proxy/test_proxy_server.py
index dd6ef04bc9..5edff6d903 100644
--- a/tests/test_litellm/proxy/test_proxy_server.py
+++ b/tests/test_litellm/proxy/test_proxy_server.py
@@ -5324,6 +5324,36 @@ async def test_window_spend_counter_redis_clean_miss_skips_stale_in_memory():
         ps.prisma_client = orig_prisma
 
 
+@pytest.mark.asyncio
+async def test_window_spend_counter_skips_invalid_window_start():
+    from litellm.caching.dual_cache import DualCache
+    from litellm.proxy.proxy_server import _init_and_increment_window_spend_counter
+
+    counter_cache = DualCache()
+
+    import litellm.proxy.proxy_server as ps
+
+    orig_counter = ps.spend_counter_cache
+    ps.spend_counter_cache = counter_cache
+    try:
+        await _init_and_increment_window_spend_counter(
+            counter_key="spend:key:key-invalid-window:window:not-a-duration",
+            entity_type="Key",
+            entity_id="key-invalid-window",
+            window_start=None,
+            increment=0.5,
+        )
+
+        assert (
+            counter_cache.in_memory_cache.get_cache(
+                key="spend:key:key-invalid-window:window:not-a-duration"
+            )
+            is None
+        )
+    finally:
+        ps.spend_counter_cache = orig_counter
+
+
 @pytest.mark.asyncio
 async def test_increment_spend_counters_finalizes_after_unreserved_increments():
     from litellm.caching.dual_cache import DualCache