Clarified error when header value is None (#3312)

Co-authored-by: Zanie Blue <contact@zanie.dev>

httpx/_utils.py

@@ -50,6 +50,8 @@ def normalize_header_value(value: str | bytes, encoding: str | None = None) -> b
     """
     if isinstance(value, bytes):
         return value
+    if not isinstance(value, str):
+        raise TypeError(f"Header value must be str or bytes, not {type(value)}")
     return value.encode(encoding or "ascii")
 
 

tests/client/test_headers.py

@@ -177,6 +177,14 @@ def test_header_does_not_exist():
         del headers["baz"]
 
 
+def test_header_with_incorrect_value():
+    with pytest.raises(
+        TypeError,
+        match=f"Header value must be str or bytes, not {type(None)}",
+    ):
+        httpx.Headers({"foo": None})  # type: ignore
+
+
 def test_host_with_auth_and_port_in_url():
     """
     The Host header should only include the hostname, or hostname:port