diff --git a/R/forecast.R b/R/forecast.R
index 94f5d5602..9472976ff 100644
--- a/R/forecast.R
+++ b/R/forecast.R
@@ -480,7 +480,7 @@ assert_forecast.forecast_nominal <- function(
 
   # forecasts need to be complete
   forecast_unit <- get_forecast_unit(forecast)
-  complete <- forecast[, .(
+  complete <- as.data.table(forecast)[, .(
     correct = test_set_equal(as.character(predicted_label), outcomes)
   ), by = forecast_unit]
 
diff --git a/R/score.R b/R/score.R
index 1674796da..b7e0c7d3f 100644
--- a/R/score.R
+++ b/R/score.R
@@ -133,6 +133,7 @@ score.forecast_nominal <- function(forecast, metrics = metrics_nominal(), ...) {
   forecast <- clean_forecast(forecast, copy = TRUE, na.omit = TRUE)
   forecast_unit <- get_forecast_unit(forecast)
   metrics <- validate_metrics(metrics)
+  forecast <- as.data.table(forecast)
 
   # transpose the forecasts that belong to the same forecast unit
   # make sure the labels and predictions are ordered in the same way