diff --git a/R/trainDI.R b/R/trainDI.R
index 505ef558..6f240fc3 100644
--- a/R/trainDI.R
+++ b/R/trainDI.R
@@ -267,6 +267,7 @@ trainDI <- function(model = NA,
       whichfold <- NA
       if(!is.null(CVtrain)&!is.null(CVtest)){
         whichfold <- as.numeric(which(lapply(CVtest,function(x){any(x==j)})==TRUE)) # index of the fold where i is held back
+        if(length(whichfold)>1){stop("a datapoint is used for testing in more than one fold. currently this option is not implemented")}
         if(length(whichfold)!=0){ # in case that a data point is never used for testing
           DItrainDist[!seq(nrow(train))%in%CVtrain[[whichfold]]] <- NA # everything that is not in the training data for i is ignored
         }