diff --git a/Sdl.Web.Tridion/Resolvers/Localization/GraphQLLocalizationResolver.cs b/Sdl.Web.Tridion/Resolvers/Localization/GraphQLLocalizationResolver.cs
index af4f05c6..b1a87ce6 100644
--- a/Sdl.Web.Tridion/Resolvers/Localization/GraphQLLocalizationResolver.cs
+++ b/Sdl.Web.Tridion/Resolvers/Localization/GraphQLLocalizationResolver.cs
@@ -67,23 +67,25 @@ public override Localization ResolveLocalization(Uri url)
                     }
 
                     string localizationId = mapping.PublicationId.ToString();
-                    if (!KnownLocalizations.TryGetValue(localizationId, out result))
+                    lock (KnownLocalizations)
                     {
-                        result = new Localization
+                        if (!KnownLocalizations.TryGetValue(localizationId, out result))
                         {
-                            Id = localizationId,
-                            Path = mapping.Path
-                        };
-                        KnownLocalizations.Add(localizationId, result);
-                    }
-                    else
-                    {
-                        // we fill in the path regardless as it may of been
-                        // a partially created localization.
-                        result.Path = mapping.Path;
+                            result = new Localization
+                            {
+                                Id = localizationId,
+                                Path = mapping.Path
+                            };
+                            KnownLocalizations.Add(localizationId, result);
+                        }
+                        else
+                        {
+                            // we fill in the path regardless as it may of been
+                            // a partially created localization.
+                            result.Path = mapping.Path;
+                        }
                     }
 
-
                     result.EnsureInitialized();
 
                     Log.Trace($"Localization for url '{url}' initialized and reports to be for Publication Id: {result.PublicationId()}, Path: {result.Path}");