diff --git a/R/check-input-helpers.R b/R/check-input-helpers.R
index 772c36527..31a4760d6 100644
--- a/R/check-input-helpers.R
+++ b/R/check-input-helpers.R
@@ -116,6 +116,7 @@ ensure_model_column <- function(data) {
 #' @inherit document_check_functions params return
 #' @keywords internal_input_check
 check_number_per_forecast <- function(data, forecast_unit) {
+  data <- ensure_data.table(data)
   data <- na.omit(data)
   # check whether there are the same number of quantiles, samples --------------
   data[, scoringutils_InternalNumCheck := length(predicted), by = forecast_unit]