diff --git a/src/NzbDrone.Core/Indexers/Newznab/Newznab.cs b/src/NzbDrone.Core/Indexers/Newznab/Newznab.cs
index 573d51937f..038e3b894e 100644
--- a/src/NzbDrone.Core/Indexers/Newznab/Newznab.cs
+++ b/src/NzbDrone.Core/Indexers/Newznab/Newznab.cs
@@ -99,7 +99,8 @@ protected static List<int> CategoryIds(List<NewznabCategory> categories)
 
             foreach (var category in categories)
             {
-                l.AddRange(CategoryIds(category.Subcategories));
+                if (category.Subcategories != null)
+                    l.AddRange(CategoryIds(category.Subcategories));
             }
             
             return l;