diff --git a/python/cog/files.py b/python/cog/files.py
index ce0bd47152..7303fb9c7e 100644
--- a/python/cog/files.py
+++ b/python/cog/files.py
@@ -14,7 +14,7 @@ def upload_file(fh: io.IOBase, output_file_prefix: Optional[str] = None) -> str:
 
     if output_file_prefix is not None:
         name = getattr(fh, "name", "output")
-        url = output_file_prefix + os.path.basename(name)
+        url = output_file_prefix.rstrip("/") + "/" + os.path.basename(name)
         resp = requests.put(url, files={"file": fh}, timeout=5)
         resp.raise_for_status()
         return url