diff --git a/omnikinverter/omnikinverter.py b/omnikinverter/omnikinverter.py
index 9ed3526c..de47c9fa 100644
--- a/omnikinverter/omnikinverter.py
+++ b/omnikinverter/omnikinverter.py
@@ -12,7 +12,11 @@
 from aiohttp.hdrs import METH_GET
 from yarl import URL
 
-from .exceptions import OmnikInverterConnectionError, OmnikInverterError
+from .exceptions import (
+    OmnikInverterAuthError,
+    OmnikInverterConnectionError,
+    OmnikInverterError,
+)
 from .models import Device, Inverter
 
 
@@ -50,6 +54,7 @@ async def request(
         Raises:
             OmnikInverterConnectionError: An error occurred while communicating
                 with the Omnik Inverter.
+            OmnikInverterAuthError: Authentication failed with the Omnik Inverter.
             OmnikInverterError: Received an unexpected response from the Omnik Inverter.
         """
         url = URL.build(scheme="http", host=self.host, path="/").join(URL(uri))
@@ -62,6 +67,12 @@ async def request(
             self.session = ClientSession()
             self._close_session = True
 
+        if self.source_type == "html" and (
+            self.username is None or self.password is None
+        ):
+            raise OmnikInverterAuthError(
+                "A username and/or password is missing from the request"
+            )
         auth = None
         if self.username and self.password:
             auth = aiohttp.BasicAuth(self.username, self.password)
diff --git a/tests/test_omnik.py b/tests/test_omnik.py
index c4551fc0..9ae92666 100644
--- a/tests/test_omnik.py
+++ b/tests/test_omnik.py
@@ -10,7 +10,7 @@
     OmnikInverterConnectionError,
     OmnikInverterWrongSourceError,
 )
-from omnikinverter.exceptions import OmnikInverterError
+from omnikinverter.exceptions import OmnikInverterAuthError, OmnikInverterError
 
 from . import load_fixtures
 
@@ -97,6 +97,25 @@ async def test_wrong_html_source(aresponses):
             assert await client.inverter()
 
 
+@pytest.mark.asyncio
+async def test_html_no_auth(aresponses):
+    """Test on html request without auth."""
+    aresponses.add(
+        "example.com",
+        "/status.html",
+        "GET",
+        aresponses.Response(
+            status=200,
+            headers={"Content-Type": "text/html"},
+        ),
+    )
+
+    async with aiohttp.ClientSession() as session:
+        client = OmnikInverter(host="example.com", source_type="html", session=session)
+        with pytest.raises(OmnikInverterAuthError):
+            assert await client.inverter()
+
+
 @pytest.mark.asyncio
 async def test_timeout(aresponses):
     """Test request timeout from Omnik Inverter."""