diff --git a/src/NzbDrone.Common/Extensions/IEnumerableExtensions.cs b/src/NzbDrone.Common/Extensions/IEnumerableExtensions.cs
index 6eb544c1d..45dbcd1d6 100644
--- a/src/NzbDrone.Common/Extensions/IEnumerableExtensions.cs
+++ b/src/NzbDrone.Common/Extensions/IEnumerableExtensions.cs
@@ -78,7 +78,8 @@ public static Dictionary<TKey, TValue> ToDictionaryIgnoreDuplicates<TItem, TKey,
             return result;
         }
 
-        public static void AddIfNotNull<TSource>(this List<TSource> source, TSource item)
+        #nullable enable
+        public static void AddIfNotNull<TSource>(this List<TSource> source, TSource? item)
         {
             if (item == null)
             {
@@ -87,6 +88,7 @@ public static void AddIfNotNull<TSource>(this List<TSource> source, TSource item
 
             source.Add(item);
         }
+        #nullable disable
 
         public static bool Empty<TSource>(this IEnumerable<TSource> source)
         {
diff --git a/src/Sonarr.Api.V5/Series/SeriesController.cs b/src/Sonarr.Api.V5/Series/SeriesController.cs
index cdadfe76a..79139b1f5 100644
--- a/src/Sonarr.Api.V5/Series/SeriesController.cs
+++ b/src/Sonarr.Api.V5/Series/SeriesController.cs
@@ -115,7 +115,7 @@ public List<SeriesResource> AllSeries(int? tvdbId, [FromQuery] SeriesSubresource
 
         if (tvdbId.HasValue)
         {
-            seriesResources.AddIfNotNull(_seriesService.FindByTvdbId(tvdbId.Value).ToResource(includeSeasonImages));
+            seriesResources.AddIfNotNull(_seriesService.FindByTvdbId(tvdbId.Value)?.ToResource(includeSeasonImages));
         }
         else
         {