diff --git a/OneDriveArchiver/IoHelper.cs b/OneDriveArchiver/IoHelper.cs
index 23eb2dd..11f2bc4 100644
--- a/OneDriveArchiver/IoHelper.cs
+++ b/OneDriveArchiver/IoHelper.cs
@@ -40,7 +40,7 @@ namespace OneDriveArchiver
                 
                 // check if we have to create either year or month folder...
                 // start with parent year folder...
-                var fullYearFolderPath = Path.Combine(basePath, fileYear);
+                var fullYearFolderPath = Path.Combine(basePath ?? throw new InvalidOperationException(), fileYear);
                 var fullMonthFolderPath = Path.Combine(Path.Combine(basePath, fileYear), fileMonth);
                 if (!Directory.Exists(fullYearFolderPath))
                 {