diff --git a/src/allmydata/storage/http_client.py b/src/allmydata/storage/http_client.py
index 9b44d2a73..9f5d6cce2 100644
--- a/src/allmydata/storage/http_client.py
+++ b/src/allmydata/storage/http_client.py
@@ -56,6 +56,7 @@ from .http_common import (
     get_content_type,
     CBOR_MIME_TYPE,
     get_spki_hash,
+    response_is_not_html,
 )
 from ..interfaces import VersionMessage
 from .common import si_b2a, si_to_human_readable
@@ -399,13 +400,17 @@ class StorageClientFactory:
         treq_client = HTTPClient(agent)
         https_url = DecodedURL().replace(scheme="https", host=nurl.host, port=nurl.port)
         swissnum = nurl.path[0].encode("ascii")
+        response_check = lambda _: None
+        if self.TEST_MODE_REGISTER_HTTP_POOL is not None:
+            response_check = response_is_not_html
+
         return StorageClient(
             https_url,
             swissnum,
             treq_client,
             pool,
             reactor,
-            self.TEST_MODE_REGISTER_HTTP_POOL is not None,
+            response_check,
         )
 
 
@@ -424,7 +429,7 @@ class StorageClient(object):
     _pool: HTTPConnectionPool
     _clock: IReactorTime
     # Are we running unit tests?
-    _test_mode: bool
+    _analyze_response: Callable[[IResponse], None] = lambda _: None
 
     def relative_url(self, path: str) -> DecodedURL:
         """Get a URL relative to the base URL."""
@@ -531,12 +536,7 @@ class StorageClient(object):
         response = await self._treq.request(
             method, url, headers=headers, timeout=timeout, **kwargs
         )
-
-        if self._test_mode and response.code != 404:
-            # We're doing API queries, HTML is never correct except in 404, but
-            # it's the default for Twisted's web server so make sure nothing
-            # unexpected happened.
-            assert get_content_type(response.headers) != "text/html"
+        self._analyze_response(response)
 
         return response
 
diff --git a/src/allmydata/storage/http_common.py b/src/allmydata/storage/http_common.py
index f16a16785..650d905e9 100644
--- a/src/allmydata/storage/http_common.py
+++ b/src/allmydata/storage/http_common.py
@@ -12,6 +12,7 @@ from cryptography.hazmat.primitives.serialization import Encoding, PublicFormat
 
 from werkzeug.http import parse_options_header
 from twisted.web.http_headers import Headers
+from twisted.web.iweb import IResponse
 
 CBOR_MIME_TYPE = "application/cbor"
 
@@ -27,6 +28,18 @@ def get_content_type(headers: Headers) -> Optional[str]:
     return content_type
 
 
+def response_is_not_html(response: IResponse) -> None:
+    """
+    During tests, this is registered so we can ensure the web server
+    doesn't give us text/html.
+
+    HTML is never correct except in 404, but it's the default for
+    Twisted's web server so we assert nothing unexpected happened.
+    """
+    if response.code != 404:
+        assert get_content_type(response.headers) != "text/html"
+
+
 def swissnum_auth_header(swissnum: bytes) -> bytes:
     """Return value for ``Authorization`` header."""
     return b"Tahoe-LAFS " + b64encode(swissnum).strip()
diff --git a/src/allmydata/test/test_storage_http.py b/src/allmydata/test/test_storage_http.py
index 2b4023bc5..30f6a527d 100644
--- a/src/allmydata/test/test_storage_http.py
+++ b/src/allmydata/test/test_storage_http.py
@@ -43,7 +43,11 @@ from testtools.matchers import Equals
 from zope.interface import implementer
 
 from .common import SyncTestCase
-from ..storage.http_common import get_content_type, CBOR_MIME_TYPE
+from ..storage.http_common import (
+    get_content_type,
+    CBOR_MIME_TYPE,
+    response_is_not_html,
+)
 from ..storage.common import si_b2a
 from ..storage.lease import LeaseInfo
 from ..storage.server import StorageServer
@@ -316,7 +320,6 @@ def result_of(d):
         + "This is probably a test design issue."
     )
 
-
 class CustomHTTPServerTests(SyncTestCase):
     """
     Tests that use a custom HTTP server.
@@ -342,7 +345,7 @@ class CustomHTTPServerTests(SyncTestCase):
             # fixed if https://github.com/twisted/treq/issues/226 were ever
             # fixed.
             clock=treq._agent._memoryReactor,
-            test_mode=True,
+            analyze_response=response_is_not_html,
         )
         self._http_server.clock = self.client._clock
 
@@ -560,7 +563,7 @@ class HttpTestFixture(Fixture):
             treq=self.treq,
             pool=None,
             clock=self.clock,
-            test_mode=True,
+            analyze_response=response_is_not_html,
         )
 
     def result_of_with_flush(self, d):
@@ -674,7 +677,7 @@ class GenericHTTPAPITests(SyncTestCase):
                 treq=StubTreq(self.http.http_server.get_resource()),
                 pool=None,
                 clock=self.http.clock,
-                test_mode=True,
+                analyze_response=response_is_not_html,
             )
         )
         with assert_fails_with_http_code(self, http.UNAUTHORIZED):