diff --git a/src/Umbraco.Core/Models/CultureImpact.cs b/src/Umbraco.Core/Models/CultureImpact.cs
index 0b035c1703..ca18985941 100644
--- a/src/Umbraco.Core/Models/CultureImpact.cs
+++ b/src/Umbraco.Core/Models/CultureImpact.cs
@@ -25,7 +25,6 @@ namespace Umbraco.Core.Models
             if (content == null) throw new ArgumentNullException(nameof(content));
             if (savingCultures == null) throw new ArgumentNullException(nameof(savingCultures));
             if (savingCultures.Length == 0) throw new ArgumentException(nameof(savingCultures));
-            if (defaultCulture == null) throw new ArgumentNullException(nameof(defaultCulture));
 
             var cultureForInvariantErrors = savingCultures.Any(x => x.InvariantEquals(defaultCulture))
                 //the default culture is being flagged for saving so use it
diff --git a/src/Umbraco.Web/Editors/ContentController.cs b/src/Umbraco.Web/Editors/ContentController.cs
index a3788e4590..a0a904b741 100644
--- a/src/Umbraco.Web/Editors/ContentController.cs
+++ b/src/Umbraco.Web/Editors/ContentController.cs
@@ -666,7 +666,7 @@ namespace Umbraco.Web.Editors
 
             //The default validation language will be either: The default languauge, else if the content is brand new and the default culture is
             // not marked to be saved, it will be the first culture in the list marked for saving.
-            var defaultCulture = _allLangs.Value.First(x => x.Value.IsDefault).Key;
+            var defaultCulture = _allLangs.Value.Values.FirstOrDefault(x => x.IsDefault)?.CultureName;
             var cultureForInvariantErrors = CultureImpact.GetCultureForInvariantErrors(
                 contentItem.PersistedContent,
                 contentItem.Variants.Where(x => x.Save).Select(x => x.Culture).ToArray(),