diff --git a/tools/lib/filereader.py b/tools/lib/filereader.py
index c499a0849..6b761ede5 100644
--- a/tools/lib/filereader.py
+++ b/tools/lib/filereader.py
@@ -5,7 +5,7 @@ DATA_PREFIX = os.getenv("DATA_PREFIX", "http://data-raw.internal/")
 
 def FileReader(fn, debug=False):
   if fn.startswith("cd:/"):
-    fn.replace("cd:/", DATA_PREFIX)
+    fn = fn.replace("cd:/", DATA_PREFIX)
   if fn.startswith("http://") or fn.startswith("https://"):
     return URLFile(fn, debug=debug)
   return open(fn, "rb")