diff --git a/httpx/_models.py b/httpx/_models.py
index 2d11888254..5bbeebbd3c 100644
--- a/httpx/_models.py
+++ b/httpx/_models.py
@@ -581,7 +581,8 @@ def encoding(self) -> str:
                 for key, value in self.raw:
                     try:
                         key.decode(encoding)
-                        value.decode(encoding)
+                        if value is not None:
+                            value.decode(encoding)
                     except UnicodeDecodeError:
                         break
                 else:
@@ -681,7 +682,12 @@ def get_list(self, key: str, split_commas: bool = False) -> typing.List[str]:
     def update(self, headers: HeaderTypes = None) -> None:  # type: ignore
         headers = Headers(headers)
         for key, value in headers.raw:
-            self[key.decode(headers.encoding)] = value.decode(headers.encoding)
+            dk = key.decode(headers.encoding)
+            if value is not None:
+                self[dk] = value.decode(headers.encoding)
+            else:
+                if dk in self:
+                    del self[dk]
 
     def copy(self) -> "Headers":
         return Headers(self, encoding=self.encoding)
diff --git a/httpx/_utils.py b/httpx/_utils.py
index 072db3f1e8..fe7017084d 100644
--- a/httpx/_utils.py
+++ b/httpx/_utils.py
@@ -51,9 +51,20 @@ def normalize_header_value(
     """
     Coerce str/bytes into a strictly byte-wise HTTP header value.
     """
-    if isinstance(value, bytes):
+    if isinstance(
+        value,
+        (
+            bytes,
+            bytearray,
+            memoryview,
+        ),
+    ):
+        return value
+    if value is None:
         return value
-    return value.encode(encoding or "ascii")
+    if isinstance(value, str):
+        return value.encode(encoding or "ascii")
+    raise NotImplementedError
 
 
 def str_query_param(value: "PrimitiveData") -> str: