diff --git a/scrapy/spidermiddlewares/referer.py b/scrapy/spidermiddlewares/referer.py
index b444e34bb..1ddfb37f4 100644
--- a/scrapy/spidermiddlewares/referer.py
+++ b/scrapy/spidermiddlewares/referer.py
@@ -261,6 +261,9 @@ _policy_classes = {p.name: p for p in (
     DefaultReferrerPolicy,
 )}
 
+# Reference: https://www.w3.org/TR/referrer-policy/#referrer-policy-empty-string
+_policy_classes[''] = NoReferrerWhenDowngradePolicy
+
 
 def _load_policy_class(policy, warning_only=False):
     """
@@ -317,8 +320,9 @@ class RefererMiddleware(object):
         policy_name = request.meta.get('referrer_policy')
         if policy_name is None:
             if isinstance(resp_or_url, Response):
-                policy_name = to_native_str(
-                    resp_or_url.headers.get('Referrer-Policy', '').decode('latin1'))
+                policy_header = resp_or_url.headers.get('Referrer-Policy')
+                if policy_header is not None:
+                    policy_name = to_native_str(policy_header.decode('latin1'))
         if policy_name is None:
             return self.default_policy()
 

lots of warnings `RuntimeWarning: Could not load referrer policy ''` when running tests
Check `tox -e py36 -- tests/test_crawl.py -s` output. @redapple is it a testing issue, or a problem with the warning? I've first noticed it in https://travis-ci.org/scrapy/scrapy/jobs/208185821 output.
diff --git a/tests/test_spidermiddleware_referer.py b/tests/test_spidermiddleware_referer.py
index b1c815876..f27f31b74 100644
--- a/tests/test_spidermiddleware_referer.py
+++ b/tests/test_spidermiddleware_referer.py
@@ -526,6 +526,13 @@ class TestPolicyHeaderPredecence003(MixinNoReferrerWhenDowngrade, TestRefererMid
     settings = {'REFERRER_POLICY': 'scrapy.spidermiddlewares.referer.OriginWhenCrossOriginPolicy'}
     resp_headers = {'Referrer-Policy': POLICY_NO_REFERRER_WHEN_DOWNGRADE.title()}
 
+class TestPolicyHeaderPredecence004(MixinNoReferrerWhenDowngrade, TestRefererMiddleware):
+    """
+    The empty string means "no-referrer-when-downgrade"
+    """
+    settings = {'REFERRER_POLICY': 'scrapy.spidermiddlewares.referer.OriginWhenCrossOriginPolicy'}
+    resp_headers = {'Referrer-Policy': ''}
+
 
 class TestReferrerOnRedirect(TestRefererMiddleware):
 

pip install -e .
parse_log_pytest
requirements.txt
pytest pytest-cov pytest-xdist pytest-mock pytest-asyncio
apt-get update apt-get install -y gcc
3.9
requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
attrs==25.3.0
Automat==24.8.1
cffi==1.17.1
constantly==23.10.4
coverage==7.8.0
cryptography==44.0.2
cssselect==1.3.0
exceptiongroup==1.2.2
execnet==2.1.1
hyperlink==21.0.0
idna==3.10
incremental==24.7.2
iniconfig==2.1.0
jmespath==1.0.1
lxml==5.3.1
packaging==24.2
parsel==1.10.0
pluggy==1.5.0
pyasn1==0.6.1
pyasn1_modules==0.4.2
pycparser==2.22
PyDispatcher==2.0.7
pyOpenSSL==25.0.0
pytest==8.3.5
pytest-asyncio==0.26.0
pytest-cov==6.0.0
pytest-mock==3.14.0
pytest-xdist==3.6.1
queuelib==1.7.0
-e git+https://github.com/scrapy/scrapy.git@d7b26edf6b419e379a7a0a425093f02cac2fcf33#egg=Scrapy
service-identity==24.2.0
six==1.17.0
tomli==2.2.1
Twisted==24.11.0
typing_extensions==4.13.0
w3lib==2.3.1
zope.interface==7.2

name: scrapy
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=3.0.16=h5eee18b_0
  - pip=25.0=py39h06a4308_0
  - python=3.9.21=he870216_1
  - readline=8.2=h5eee18b_0
  - setuptools=75.8.0=py39h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - tzdata=2025a=h04d1e81_0
  - wheel=0.45.1=py39h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - attrs==25.3.0
      - automat==24.8.1
      - cffi==1.17.1
      - constantly==23.10.4
      - coverage==7.8.0
      - cryptography==44.0.2
      - cssselect==1.3.0
      - exceptiongroup==1.2.2
      - execnet==2.1.1
      - hyperlink==21.0.0
      - idna==3.10
      - incremental==24.7.2
      - iniconfig==2.1.0
      - jmespath==1.0.1
      - lxml==5.3.1
      - packaging==24.2
      - parsel==1.10.0
      - pluggy==1.5.0
      - pyasn1==0.6.1
      - pyasn1-modules==0.4.2
      - pycparser==2.22
      - pydispatcher==2.0.7
      - pyopenssl==25.0.0
      - pytest==8.3.5
      - pytest-asyncio==0.26.0
      - pytest-cov==6.0.0
      - pytest-mock==3.14.0
      - pytest-xdist==3.6.1
      - queuelib==1.7.0
      - service-identity==24.2.0
      - six==1.17.0
      - tomli==2.2.1
      - twisted==24.11.0
      - typing-extensions==4.13.0
      - w3lib==2.3.1
      - zope-interface==7.2
prefix: /opt/conda/envs/scrapy

tests/test_spidermiddleware_referer.py::TestPolicyHeaderPredecence004::test
tests/test_spidermiddleware_referer.py::TestRefererMiddleware::test tests/test_spidermiddleware_referer.py::TestRefererMiddlewareDefault::test tests/test_spidermiddleware_referer.py::TestSettingsNoReferrer::test tests/test_spidermiddleware_referer.py::TestSettingsNoReferrerWhenDowngrade::test tests/test_spidermiddleware_referer.py::TestSettingsSameOrigin::test tests/test_spidermiddleware_referer.py::TestSettingsOrigin::test tests/test_spidermiddleware_referer.py::TestSettingsStrictOrigin::test tests/test_spidermiddleware_referer.py::TestSettingsOriginWhenCrossOrigin::test tests/test_spidermiddleware_referer.py::TestSettingsStrictOriginWhenCrossOrigin::test tests/test_spidermiddleware_referer.py::TestSettingsUnsafeUrl::test tests/test_spidermiddleware_referer.py::TestSettingsCustomPolicy::test tests/test_spidermiddleware_referer.py::TestRequestMetaDefault::test tests/test_spidermiddleware_referer.py::TestRequestMetaNoReferrer::test tests/test_spidermiddleware_referer.py::TestRequestMetaNoReferrerWhenDowngrade::test tests/test_spidermiddleware_referer.py::TestRequestMetaSameOrigin::test tests/test_spidermiddleware_referer.py::TestRequestMetaOrigin::test tests/test_spidermiddleware_referer.py::TestRequestMetaSrictOrigin::test tests/test_spidermiddleware_referer.py::TestRequestMetaOriginWhenCrossOrigin::test tests/test_spidermiddleware_referer.py::TestRequestMetaStrictOriginWhenCrossOrigin::test tests/test_spidermiddleware_referer.py::TestRequestMetaUnsafeUrl::test tests/test_spidermiddleware_referer.py::TestRequestMetaPredecence001::test tests/test_spidermiddleware_referer.py::TestRequestMetaPredecence002::test tests/test_spidermiddleware_referer.py::TestRequestMetaPredecence003::test tests/test_spidermiddleware_referer.py::TestRequestMetaSettingFallback::test tests/test_spidermiddleware_referer.py::TestSettingsPolicyByName::test_invalid_name tests/test_spidermiddleware_referer.py::TestSettingsPolicyByName::test_valid_name tests/test_spidermiddleware_referer.py::TestSettingsPolicyByName::test_valid_name_casevariants tests/test_spidermiddleware_referer.py::TestPolicyHeaderPredecence001::test tests/test_spidermiddleware_referer.py::TestPolicyHeaderPredecence002::test tests/test_spidermiddleware_referer.py::TestPolicyHeaderPredecence003::test tests/test_spidermiddleware_referer.py::TestReferrerOnRedirect::test tests/test_spidermiddleware_referer.py::TestReferrerOnRedirectNoReferrer::test tests/test_spidermiddleware_referer.py::TestReferrerOnRedirectSameOrigin::test tests/test_spidermiddleware_referer.py::TestReferrerOnRedirectStrictOrigin::test tests/test_spidermiddleware_referer.py::TestReferrerOnRedirectOriginWhenCrossOrigin::test tests/test_spidermiddleware_referer.py::TestReferrerOnRedirectStrictOriginWhenCrossOrigin::test
scrapy/spidermiddlewares/referer.py
scrapy/spidermiddlewares/referer.py