diff --git a/src/openai/_base_client.py b/src/openai/_base_client.py
index c2c2db5f49..fa0373fa80 100644
--- a/src/openai/_base_client.py
+++ b/src/openai/_base_client.py
@@ -420,8 +420,8 @@ def _prepare_url(self, url: str) -> URL:
         # Copied from httpx's `_merge_url` method.
         merge_url = URL(url)
         if merge_url.is_relative_url:
-            merge_raw_path = self.base_url.raw_path + merge_url.raw_path.lstrip(b"/")
-            return self.base_url.copy_with(raw_path=merge_raw_path)
+            merge_raw_path = self._base_url.raw_path + merge_url.raw_path.lstrip(b"/")
+            return self._base_url.copy_with(raw_path=merge_raw_path)
 
         return merge_url