diff --git a/setup.py b/setup.py
index 4cf9073b..b82dcafb 100755
--- a/setup.py
+++ b/setup.py
@@ -18,7 +18,7 @@
     "sphinx>=1.4.0",
 ]
 
-async_require = ["httpx"]
+async_require = ["httpx>=0.15.0"]
 
 xmlsec_require = [
     "xmlsec>=0.6.1",
diff --git a/src/zeep/transports.py b/src/zeep/transports.py
index 0841e2f1..ffde356b 100644
--- a/src/zeep/transports.py
+++ b/src/zeep/transports.py
@@ -218,7 +218,7 @@ async def post(self, address, message, headers):
         self.logger.debug("HTTP Post to %s:\n%s", address, message)
         response = await self.client.post(
             address,
-            data=message,
+            content=message,
             headers=headers,
         )
         self.logger.debug(
diff --git a/tests/test_async_transport.py b/tests/test_async_transport.py
index f5e8d1b0..08f2b0f4 100644
--- a/tests/test_async_transport.py
+++ b/tests/test_async_transport.py
@@ -19,7 +19,7 @@ def test_load(httpx_mock):
     cache = stub(get=lambda url: None, add=lambda url, content: None)
     transport = AsyncTransport(cache=cache)
 
-    httpx_mock.add_response(url="http://tests.python-zeep.org/test.xml", data="x")
+    httpx_mock.add_response(url="http://tests.python-zeep.org/test.xml", content="x")
     result = transport.load("http://tests.python-zeep.org/test.xml")
     assert result == b"x"
 
@@ -30,7 +30,7 @@ def test_load_cache(httpx_mock):
     cache = InMemoryCache()
     transport = AsyncTransport(cache=cache)
 
-    httpx_mock.add_response(url="http://tests.python-zeep.org/test.xml", data="x")
+    httpx_mock.add_response(url="http://tests.python-zeep.org/test.xml", content="x")
     result = transport.load("http://tests.python-zeep.org/test.xml")
     assert result == b"x"
 
@@ -45,7 +45,7 @@ async def test_post(httpx_mock: HTTPXMock):
 
     envelope = etree.Element("Envelope")
 
-    httpx_mock.add_response(url="http://tests.python-zeep.org/test.xml", data="x")
+    httpx_mock.add_response(url="http://tests.python-zeep.org/test.xml", content="x")
     result = await transport.post_xml(
         "http://tests.python-zeep.org/test.xml", envelope=envelope, headers={}
     )
@@ -67,7 +67,7 @@ async def test_http_error(httpx_mock: HTTPXMock):
     transport = AsyncTransport()
 
     httpx_mock.add_response(
-        url="http://tests.python-zeep.org/test.xml", data="x", status_code=500
+        url="http://tests.python-zeep.org/test.xml", content="x", status_code=500
     )
     with pytest.raises(exceptions.TransportError) as exc:
         transport.load("http://tests.python-zeep.org/test.xml")