diff --git a/google/oauth2/_client.py b/google/oauth2/_client.py
index f92b097..4cf7a7f 100644
--- a/google/oauth2/_client.py
+++ b/google/oauth2/_client.py
@@ -103,7 +103,11 @@
     # occurs.
     while True:
         response = request(method="POST", url=token_uri, headers=headers, body=body)
-        response_body = response.data.decode("utf-8")
+        response_body = (
+            response.data.decode("utf-8")
+            if hasattr(response.data, "decode")
+            else response.data
+        )
         response_data = json.loads(response_body)
 
         if response.status == http_client.OK: