diff --git a/httpx/_client.py b/httpx/_client.py
index 7248f87..2c56edc 100644
--- a/httpx/_client.py
+++ b/httpx/_client.py
@@ -322,6 +322,10 @@ class BaseClient:
 
         url = URL(location, allow_relative=True)
 
+        # Check that we can handle the scheme
+        if url.scheme and url.scheme not in ("http", "https"):
+            raise InvalidURL(f'Scheme "{url.scheme}" not supported.')
+
         # Handle malformed 'Location' headers that are "absolute" form, have no host.
         # See: https://github.com/encode/httpx/issues/771
         if url.scheme and not url.host:

KeyError occurs when redirect to custom scheme(ex. market://)
# Information

OS platform : mac OS
Python version : 3.7.2
Installed dependencies and versions : `httpx==0.9.3`
Code snippet
```
    @property
    def port(self) -> int:
        port = self._uri_reference.port
        if port is None:
            return {"https": 443, "http": 80}[self.scheme]
        return int(port)
```

Error traceback
```
[2020-02-24 14:57:08 +0900] [82150] [ERROR] Exception
Traceback (most recent call last):
  File "/Users/sym/.pyenv/versions/airbridge-ads-was/lib/python3.7/site-packages/sanic/testing.py", line 120, in _collect_response
    method, url, *request_args, **request_kwargs
  File "/Users/sym/.pyenv/versions/airbridge-ads-was/lib/python3.7/site-packages/sanic/testing.py", line 41, in _local_request
    url, verify=False, *args, **kwargs
  File "/Users/sym/.pyenv/versions/airbridge-ads-was/lib/python3.7/site-packages/httpx/client.py", line 671, in get
    trust_env=trust_env,
  File "/Users/sym/.pyenv/versions/airbridge-ads-was/lib/python3.7/site-packages/httpx/client.py", line 268, in request
    trust_env=trust_env,
  File "/Users/sym/.pyenv/versions/airbridge-ads-was/lib/python3.7/site-packages/httpx/client.py", line 410, in send
    allow_redirects=allow_redirects,
  File "/Users/sym/.pyenv/versions/airbridge-ads-was/lib/python3.7/site-packages/httpx/client.py", line 478, in send_handling_redirects
    request = self.build_redirect_request(request, response)
  File "/Users/sym/.pyenv/versions/airbridge-ads-was/lib/python3.7/site-packages/httpx/client.py", line 500, in build_redirect_request
    headers = self.redirect_headers(request, url, method)
  File "/Users/sym/.pyenv/versions/airbridge-ads-was/lib/python3.7/site-packages/httpx/client.py", line 555, in redirect_headers
    if url.origin != request.url.origin:
  File "/Users/sym/.pyenv/versions/airbridge-ads-was/lib/python3.7/site-packages/httpx/models.py", line 215, in origin
    return Origin(self)
  File "/Users/sym/.pyenv/versions/airbridge-ads-was/lib/python3.7/site-packages/httpx/models.py", line 287, in __init__
    self.port = url.port
  File "/Users/sym/.pyenv/versions/airbridge-ads-was/lib/python3.7/site-packages/httpx/models.py", line 165, in port
    return {"https": 443, "http": 80}[self.scheme]
KeyError: 'market'
[2020-02-24 14:57:08 +0900] [82150] [INFO] Starting worker [82150]
[2020-02-24 14:57:08 +0900] [82150] [INFO] Stopping worker [82150]
[2020-02-24 14:57:08 +0900] [82150] [INFO] Server Stopped
```

# Description
i'm using sanic and sanic uses httpx to test web request.
when i make a redirect response which goes to "market://details?id=~~" (android market url)", KeyError occurred.
I think it is associated with `port` property method.
Is this the intended behavior?

Thank you.
diff --git a/tests/client/test_redirects.py b/tests/client/test_redirects.py
index e91bb7e..fa5ae4e 100644
--- a/tests/client/test_redirects.py
+++ b/tests/client/test_redirects.py
@@ -8,6 +8,7 @@ import pytest
 from httpx import (
     URL,
     AsyncClient,
+    InvalidURL,
     NotRedirectResponse,
     RequestBodyUnavailable,
     TooManyRedirects,
@@ -140,6 +141,17 @@ class MockDispatch(httpcore.AsyncHTTPTransport):
             else:
                 return b"HTTP/1.1", 200, b"OK", [], ByteStream(b"Hello, world!")
 
+        elif path == b"/redirect_custom_scheme":
+            status_code = codes.MOVED_PERMANENTLY
+            headers = [(b"location", b"market://details?id=42")]
+            return (
+                b"HTTP/1.1",
+                status_code,
+                b"Moved Permanently",
+                headers,
+                ByteStream(b""),
+            )
+
         return b"HTTP/1.1", 200, b"OK", [], ByteStream(b"Hello, world!")
 
 
@@ -431,3 +443,11 @@ async def test_redirect_cookie_behavior():
     response = await client.get("https://example.com/")
     assert response.url == "https://example.com/"
     assert response.text == "Not logged in"
+
+
+@pytest.mark.usefixtures("async_environment")
+async def test_redirect_custom_scheme():
+    client = AsyncClient(dispatch=MockDispatch())
+    with pytest.raises(InvalidURL) as e:
+        await client.post("https://example.org/redirect_custom_scheme")
+    assert str(e.value) == 'Scheme "market" not supported.'

pip install -e .
parse_log_pytest
requirements.txt
pytest
3.9
requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
aspy.refactor-imports==3.0.2
async-generator==1.10
attrs==25.3.0
autoflake==2.3.1
babel==2.17.0
backports.tarfile==1.2.0
backrefs==5.8
black==25.1.0
brotlipy==0.7.0
certifi==2025.1.31
cffi==1.17.1
chardet==3.0.4
charset-normalizer==3.4.1
click==8.1.8
colorama==0.4.6
coverage==7.8.0
cryptography==44.0.2
docutils==0.21.2
exceptiongroup==1.2.2
flake8==7.2.0
flake8-bugbear==24.12.12
flake8-pie==0.16.0
ghp-import==2.1.0
h11==0.9.0
h2==3.2.0
hpack==3.0.0
hstspreload==2025.1.1
httpcore==0.9.1
-e git+https://github.com/encode/httpx.git@21d7e16559d9360ae3a5c5cfd23bab8bb85ee4a8#egg=httpx
hyperframe==5.2.0
id==1.5.0
idna==2.10
importlib_metadata==8.6.1
iniconfig==2.1.0
isort==6.0.1
jaraco.classes==3.4.0
jaraco.context==6.0.1
jaraco.functools==4.1.0
jeepney==0.9.0
Jinja2==3.1.6
keyring==25.6.0
Markdown==3.7
markdown-it-py==3.0.0
MarkupSafe==3.0.2
mccabe==0.7.0
mdurl==0.1.2
mergedeep==1.3.4
mkautodoc==0.2.0
mkdocs==1.6.1
mkdocs-get-deps==0.2.0
mkdocs-material==9.6.10
mkdocs-material-extensions==1.3.1
more-itertools==10.6.0
mypy==1.15.0
mypy-extensions==1.0.0
nh3==0.2.21
outcome==1.3.0.post0
packaging==24.2
paginate==0.5.7
pathspec==0.12.1
platformdirs==4.3.7
pluggy==1.5.0
pycodestyle==2.13.0
pycparser==2.22
pyflakes==3.3.2
Pygments==2.19.1
pymdown-extensions==10.14.3
pytest==8.3.5
pytest-asyncio==0.26.0
pytest-cov==6.0.0
pytest-trio==0.8.0
python-dateutil==2.9.0.post0
PyYAML==6.0.2
pyyaml_env_tag==0.1
readme_renderer==44.0
requests==2.32.3
requests-toolbelt==1.0.0
rfc3986==1.5.0
rich==14.0.0
SecretStorage==3.3.3
seed-isort-config==2.2.0
six==1.17.0
sniffio==1.3.1
sortedcontainers==2.4.0
tomli==2.2.1
trio==0.29.0
trio-typing==0.10.0
trustme==1.2.1
twine==6.1.0
typing_extensions==4.13.0
urllib3==2.3.0
uvicorn==0.34.0
watchdog==6.0.0
zipp==3.21.0

name: httpx
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:
      - aspy-refactor-imports==3.0.2
      - async-generator==1.10
      - attrs==25.3.0
      - autoflake==2.3.1
      - babel==2.17.0
      - backports-tarfile==1.2.0
      - backrefs==5.8
      - black==25.1.0
      - brotlipy==0.7.0
      - certifi==2025.1.31
      - cffi==1.17.1
      - chardet==3.0.4
      - charset-normalizer==3.4.1
      - click==8.1.8
      - colorama==0.4.6
      - coverage==7.8.0
      - cryptography==44.0.2
      - docutils==0.21.2
      - exceptiongroup==1.2.2
      - flake8==7.2.0
      - flake8-bugbear==24.12.12
      - flake8-pie==0.16.0
      - ghp-import==2.1.0
      - h11==0.9.0
      - h2==3.2.0
      - hpack==3.0.0
      - hstspreload==2025.1.1
      - httpcore==0.9.1
      - hyperframe==5.2.0
      - id==1.5.0
      - idna==2.10
      - importlib-metadata==8.6.1
      - iniconfig==2.1.0
      - isort==6.0.1
      - jaraco-classes==3.4.0
      - jaraco-context==6.0.1
      - jaraco-functools==4.1.0
      - jeepney==0.9.0
      - jinja2==3.1.6
      - keyring==25.6.0
      - markdown==3.7
      - markdown-it-py==3.0.0
      - markupsafe==3.0.2
      - mccabe==0.7.0
      - mdurl==0.1.2
      - mergedeep==1.3.4
      - mkautodoc==0.2.0
      - mkdocs==1.6.1
      - mkdocs-get-deps==0.2.0
      - mkdocs-material==9.6.10
      - mkdocs-material-extensions==1.3.1
      - more-itertools==10.6.0
      - mypy==1.15.0
      - mypy-extensions==1.0.0
      - nh3==0.2.21
      - outcome==1.3.0.post0
      - packaging==24.2
      - paginate==0.5.7
      - pathspec==0.12.1
      - platformdirs==4.3.7
      - pluggy==1.5.0
      - pycodestyle==2.13.0
      - pycparser==2.22
      - pyflakes==3.3.2
      - pygments==2.19.1
      - pymdown-extensions==10.14.3
      - pytest==8.3.5
      - pytest-asyncio==0.26.0
      - pytest-cov==6.0.0
      - pytest-trio==0.8.0
      - python-dateutil==2.9.0.post0
      - pyyaml==6.0.2
      - pyyaml-env-tag==0.1
      - readme-renderer==44.0
      - requests==2.32.3
      - requests-toolbelt==1.0.0
      - rfc3986==1.5.0
      - rich==14.0.0
      - secretstorage==3.3.3
      - seed-isort-config==2.2.0
      - six==1.17.0
      - sniffio==1.3.1
      - sortedcontainers==2.4.0
      - tomli==2.2.1
      - trio==0.29.0
      - trio-typing==0.10.0
      - trustme==1.2.1
      - twine==6.1.0
      - typing-extensions==4.13.0
      - urllib3==2.3.0
      - uvicorn==0.34.0
      - watchdog==6.0.0
      - zipp==3.21.0
prefix: /opt/conda/envs/httpx

tests/client/test_redirects.py::test_redirect_custom_scheme[asyncio] tests/client/test_redirects.py::test_redirect_custom_scheme[trio]
tests/client/test_redirects.py::test_no_redirect[asyncio] tests/client/test_redirects.py::test_no_redirect[trio] tests/client/test_redirects.py::test_redirect_301[asyncio] tests/client/test_redirects.py::test_redirect_301[trio] tests/client/test_redirects.py::test_redirect_302[asyncio] tests/client/test_redirects.py::test_redirect_302[trio] tests/client/test_redirects.py::test_redirect_303[asyncio] tests/client/test_redirects.py::test_redirect_303[trio] tests/client/test_redirects.py::test_disallow_redirects[asyncio] tests/client/test_redirects.py::test_disallow_redirects[trio] tests/client/test_redirects.py::test_relative_redirect[asyncio] tests/client/test_redirects.py::test_relative_redirect[trio] tests/client/test_redirects.py::test_malformed_redirect[asyncio] tests/client/test_redirects.py::test_malformed_redirect[trio] tests/client/test_redirects.py::test_no_scheme_redirect[asyncio] tests/client/test_redirects.py::test_no_scheme_redirect[trio] tests/client/test_redirects.py::test_fragment_redirect[asyncio] tests/client/test_redirects.py::test_fragment_redirect[trio] tests/client/test_redirects.py::test_multiple_redirects[asyncio] tests/client/test_redirects.py::test_multiple_redirects[trio] tests/client/test_redirects.py::test_too_many_redirects[asyncio] tests/client/test_redirects.py::test_too_many_redirects[trio] tests/client/test_redirects.py::test_too_many_redirects_calling_next[asyncio] tests/client/test_redirects.py::test_too_many_redirects_calling_next[trio] tests/client/test_redirects.py::test_redirect_loop[asyncio] tests/client/test_redirects.py::test_redirect_loop[trio] tests/client/test_redirects.py::test_cross_domain_redirect[asyncio] tests/client/test_redirects.py::test_cross_domain_redirect[trio] tests/client/test_redirects.py::test_same_domain_redirect[asyncio] tests/client/test_redirects.py::test_same_domain_redirect[trio] tests/client/test_redirects.py::test_body_redirect[asyncio] tests/client/test_redirects.py::test_body_redirect[trio] tests/client/test_redirects.py::test_no_body_redirect[asyncio] tests/client/test_redirects.py::test_no_body_redirect[trio] tests/client/test_redirects.py::test_can_stream_if_no_redirect[asyncio] tests/client/test_redirects.py::test_can_stream_if_no_redirect[trio] tests/client/test_redirects.py::test_cannot_redirect_streaming_body[asyncio] tests/client/test_redirects.py::test_cannot_redirect_streaming_body[trio] tests/client/test_redirects.py::test_cross_subdomain_redirect[asyncio] tests/client/test_redirects.py::test_cross_subdomain_redirect[trio] tests/client/test_redirects.py::test_redirect_cookie_behavior[asyncio] tests/client/test_redirects.py::test_redirect_cookie_behavior[trio]
httpx/_client.py
httpx/_client.py