diff --git a/src/anthropic/_client.py b/src/anthropic/_client.py
index 76b840608..3f527902c 100644
--- a/src/anthropic/_client.py
+++ b/src/anthropic/_client.py
@@ -211,8 +211,8 @@ def __init__(
             or profile is not None
         )
         if not has_explicit_credential:
-            api_key = os.environ.get("ANTHROPIC_API_KEY")
-            auth_token = os.environ.get("ANTHROPIC_AUTH_TOKEN")
+            api_key = os.environ.get("ANTHROPIC_API_KEY") or None
+            auth_token = os.environ.get("ANTHROPIC_AUTH_TOKEN") or None
         self.api_key = api_key
         self.auth_token = auth_token
         # --- end credentials support ---
@@ -628,8 +628,8 @@ def __init__(
             or profile is not None
         )
         if not has_explicit_credential:
-            api_key = os.environ.get("ANTHROPIC_API_KEY")
-            auth_token = os.environ.get("ANTHROPIC_AUTH_TOKEN")
+            api_key = os.environ.get("ANTHROPIC_API_KEY") or None
+            auth_token = os.environ.get("ANTHROPIC_AUTH_TOKEN") or None
         self.api_key = api_key
         self.auth_token = auth_token
         # --- end credentials support ---
diff --git a/tests/test_client.py b/tests/test_client.py
index b6ebf9e4d..61fd0994b 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -434,6 +434,17 @@ def test_validate_headers(self) -> None:
         request2 = client2._build_request(FinalRequestOptions(method="get", url="/foo", headers={"X-Api-Key": Omit()}))
         assert request2.headers.get("X-Api-Key") is None
 
+    def test_empty_string_env_credentials_treated_as_absent(self) -> None:
+        # An empty-string env var must not be used as a credential — it would
+        # produce a malformed "Authorization: Bearer " header rejected by h11.
+        with mock.patch("anthropic._client.default_credentials", return_value=None):
+            with update_env(ANTHROPIC_API_KEY="", ANTHROPIC_AUTH_TOKEN=""):
+                client = Anthropic(base_url=base_url, _strict_response_validation=True)
+
+        assert client.api_key is None
+        assert client.auth_token is None
+        assert client.auth_headers == {}
+
     def test_default_query_option(self) -> None:
         client = Anthropic(
             base_url=base_url, api_key=api_key, _strict_response_validation=True, default_query={"query_param": "bar"}
@@ -1546,6 +1557,17 @@ def test_validate_headers(self) -> None:
         request2 = client2._build_request(FinalRequestOptions(method="get", url="/foo", headers={"X-Api-Key": Omit()}))
         assert request2.headers.get("X-Api-Key") is None
 
+    def test_empty_string_env_credentials_treated_as_absent(self) -> None:
+        # An empty-string env var must not be used as a credential — it would
+        # produce a malformed "Authorization: Bearer " header rejected by h11.
+        with mock.patch("anthropic._client.default_credentials", return_value=None):
+            with update_env(ANTHROPIC_API_KEY="", ANTHROPIC_AUTH_TOKEN=""):
+                client = AsyncAnthropic(base_url=base_url, _strict_response_validation=True)
+
+        assert client.api_key is None
+        assert client.auth_token is None
+        assert client.auth_headers == {}
+
     async def test_default_query_option(self) -> None:
         client = AsyncAnthropic(
             base_url=base_url, api_key=api_key, _strict_response_validation=True, default_query={"query_param": "bar"}