diff --git a/src/otoole/results/result_package.py b/src/otoole/results/result_package.py
index 991b3fe2..255f6847 100644
--- a/src/otoole/results/result_package.py
+++ b/src/otoole/results/result_package.py
@@ -858,7 +858,7 @@ def discount_factor(
     if regions and years:
         discount_rate["YEAR"] = [years]
         discount_factor = discount_rate.explode("YEAR").reset_index(level="REGION")
-        discount_factor["YEAR"] = discount_factor["YEAR"].astype(int)
+        discount_factor["YEAR"] = discount_factor["YEAR"].astype("int64")
         discount_factor["NUM"] = discount_factor["YEAR"] - discount_factor["YEAR"].min()
         discount_factor["RATE"] = discount_factor["VALUE"] + 1
         discount_factor["VALUE"] = (