diff --git a/Sources/Foundation/NSCalendar.swift b/Sources/Foundation/NSCalendar.swift
index 20774d22d9..465912e6a5 100644
--- a/Sources/Foundation/NSCalendar.swift
+++ b/Sources/Foundation/NSCalendar.swift
@@ -59,6 +59,9 @@ extension NSCalendar {
             case .republicOfChina: self = .republicOfChina
             case .islamicTabular: self = .islamicTabular
             case .islamicUmmAlQura: self = .islamicUmmAlQura
+            @unknown default:
+                // Default to gregorian if `FoundationEssentials.Calendar` adds more calendar that we do not support here yet
+                self = .gregorian
             }
         }