diff --git a/submission.R b/submission.R
index 604abc1..6f12557 100644
--- a/submission.R
+++ b/submission.R
@@ -16,7 +16,7 @@
 # List your packages here. Don't forget to update packages.R!
 library(dplyr) # as an example, not used here
 
-clean_df <- function(df, background_df){
+clean_df <- function(df, background_df=NULL){
   # Preprocess the input dataframe to feed the model.
   ### If no cleaning is done (e.g. if all the cleaning is done in a pipeline) leave only the "return df" command