diff --git a/src/GmRelay.Web/Services/SubscriptionNotFoundException.cs b/src/GmRelay.Web/Services/SubscriptionNotFoundException.cs
new file mode 100644
index 0000000..cf573b0
--- /dev/null
+++ b/src/GmRelay.Web/Services/SubscriptionNotFoundException.cs
@@ -0,0 +1,6 @@
+namespace GmRelay.Web.Services;
+
+public sealed class SubscriptionNotFoundException : Exception
+{
+    public SubscriptionNotFoundException() : base("Calendar subscription not found.") { }
+}