diff --git a/src/openai/_base_client.py b/src/openai/_base_client.py
index 58490e4430..9339a367b9 100644
--- a/src/openai/_base_client.py
+++ b/src/openai/_base_client.py
@@ -436,7 +436,7 @@ def _build_headers(self, options: FinalRequestOptions, *, retries_taken: int = 0
         self._validate_headers(headers_dict, custom_headers)
 
         # headers are case-insensitive while dictionaries are not.
-        headers = httpx.Headers(headers_dict)
+        headers = httpx.Headers(headers_dict, encoding="utf-8")
 
         idempotency_header = self._idempotency_header
         if idempotency_header and options.idempotency_key and idempotency_header not in headers: