diff --git a/R/irg.R b/R/irg.R
index 5909d5f9f4ad0fda9bdc2293e0b2868e1acf0a3e..bc7df796f1d280a059a95d0d141bdf81cf63d571 100644
--- a/R/irg.R
+++ b/R/irg.R
@@ -85,7 +85,9 @@ calc_irg <-
 			check_col(DT, id, 'id')
 			check_col(DT, year, 'year')
 
-			DT[, irg := (irg - min(irg)) / (max(irg) - min(irg)),
+			DT[, irg :=
+				 	(irg - min(irg, na.rm = TRUE)) /
+				 	(max(irg, na.rm = TRUE) - min(irg, na.rm = TRUE)),
 				 by = c(id, year)]
 		}