diff --git a/allgo/__init__.py b/allgo/__init__.py
index 3e901a974117ccfbcad65769e606e469f2533f78..b881109cb786d0fe81f5df5e1f3c2cc098eab304 100644
--- a/allgo/__init__.py
+++ b/allgo/__init__.py
@@ -438,7 +438,7 @@ class Client:
 
         # method with os.fdopen(os.open(outfilepath, mode), 'w') doesn't exist
         if os.path.isfile(outfilepath) and not force:
-            msg = "file {} exists."
+            msg = "file {} exists.".format(outfilepath)
             raise OSError(errno.EEXIST, msg)
 
         try: