diff --git a/httpx/_models.py b/httpx/_models.py
index 17ba955890..5b6a9b6571 100644
--- a/httpx/_models.py
+++ b/httpx/_models.py
@@ -71,8 +71,12 @@ def __init__(self, url: URLTypes = "", params: QueryParamTypes = None) -> None:
                 # We don't want to normalize relative URLs, since doing so
                 # removes any leading `../` portion.
                 self._uri_reference = self._uri_reference.normalize()
-        else:
+        elif isinstance(url, URL):
             self._uri_reference = url._uri_reference
+        else:
+            raise TypeError(
+                f"Invalid type for url.  Expected str or httpx.URL, got {type(url)}"
+            )
 
         # Add any query parameters, merging with any in the URL if needed.
         if params:
diff --git a/tests/models/test_url.py b/tests/models/test_url.py
index fa75e556a6..8d34a75a79 100644
--- a/tests/models/test_url.py
+++ b/tests/models/test_url.py
@@ -204,3 +204,11 @@ def test_url_copywith_for_userinfo():
 def test_url_invalid():
     with pytest.raises(httpx.InvalidURL):
         httpx.URL("https://😇/")
+
+
+def test_url_invalid_type():
+    class ExternalURLClass:  # representing external URL class
+        pass
+
+    with pytest.raises(TypeError):
+        httpx.URL(ExternalURLClass())  # type: ignore