diff --git a/R/check-input-helpers.R b/R/check-input-helpers.R
index 31a4760d6..0b1c820e2 100644
--- a/R/check-input-helpers.R
+++ b/R/check-input-helpers.R
@@ -113,11 +113,13 @@ ensure_model_column <- function(data) {
 #' If the number of quantiles or samples is the same for all forecasts, it
 #' returns TRUE and a string with an error message otherwise.
 #' @param forecast_unit Character vector denoting the unit of a single forecast.
+#' @importFrom checkmate assert_subset
 #' @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)
+  assert_subset(forecast_unit, colnames(data))
   # check whether there are the same number of quantiles, samples --------------
   data[, scoringutils_InternalNumCheck := length(predicted), by = forecast_unit]
   n <- unique(data$scoringutils_InternalNumCheck)