diff --git a/litellm/proxy/guardrails/guardrail_registry.py b/litellm/proxy/guardrails/guardrail_registry.py
index 64e86103dbd..0dfcad7d859 100644
--- a/litellm/proxy/guardrails/guardrail_registry.py
+++ b/litellm/proxy/guardrails/guardrail_registry.py
@@ -448,9 +448,17 @@ class InMemoryGuardrailHandler:
             if isinstance(default_on_raw, str) and default_on_raw.startswith(
                 "os.environ/"
             ):
+                resolved_default_on = get_secret_bool(default_on_raw)
+                if resolved_default_on is None:
+                    verbose_proxy_logger.warning(
+                        "Guardrail default_on env var %s could not be resolved "
+                        "to a bool (unset or value not 'true'/'false'); "
+                        "defaulting to False",
+                        default_on_raw,
+                    )
                 litellm_params_data = {
                     **litellm_params_data,
-                    "default_on": get_secret_bool(default_on_raw),
+                    "default_on": resolved_default_on,
                 }
             litellm_params = LitellmParams(**litellm_params_data)
         else:
diff --git a/tests/test_litellm/proxy/guardrails/test_guardrail_registry.py b/tests/test_litellm/proxy/guardrails/test_guardrail_registry.py
index df071892c0f..fbe29812ec5 100644
--- a/tests/test_litellm/proxy/guardrails/test_guardrail_registry.py
+++ b/tests/test_litellm/proxy/guardrails/test_guardrail_registry.py
@@ -412,16 +412,38 @@ def test_initialize_guardrail_resolves_default_on_env_var_false(monkeypatch):
     assert callback.default_on is False
 
 
-def test_initialize_guardrail_unsets_default_on_env_var_falls_back_false(monkeypatch):
+def test_initialize_guardrail_unsets_default_on_env_var_falls_back_false(
+    monkeypatch, caplog
+):
     captured: dict = {}
     monkeypatch.delenv("LITELLM_TEST_DEFAULT_ON_MISSING", raising=False)
-    handler = _init_handler_with_fake_initializer(
-        captured, "os.environ/LITELLM_TEST_DEFAULT_ON_MISSING"
-    )
+    import logging
+
+    with caplog.at_level(logging.WARNING, logger="LiteLLM Proxy"):
+        handler = _init_handler_with_fake_initializer(
+            captured, "os.environ/LITELLM_TEST_DEFAULT_ON_MISSING"
+        )
 
     assert captured["default_on"] is False
     callback = handler.guardrail_id_to_custom_guardrail["default-on-env-var"]
     assert callback.default_on is False
+    assert "LITELLM_TEST_DEFAULT_ON_MISSING" in caplog.text
+
+
+def test_initialize_guardrail_unrecognized_default_on_env_value_warns(
+    monkeypatch, caplog
+):
+    captured: dict = {}
+    monkeypatch.setenv("LITELLM_TEST_DEFAULT_ON_UNRECOGNIZED", "yes")
+    import logging
+
+    with caplog.at_level(logging.WARNING, logger="LiteLLM Proxy"):
+        _init_handler_with_fake_initializer(
+            captured, "os.environ/LITELLM_TEST_DEFAULT_ON_UNRECOGNIZED"
+        )
+
+    assert captured["default_on"] is False
+    assert "LITELLM_TEST_DEFAULT_ON_UNRECOGNIZED" in caplog.text
 
 
 def test_initialize_guardrail_non_prefixed_bool_string_not_env_resolved(monkeypatch):