diff --git a/src/Umbraco.Core/CoreRuntime.cs b/src/Umbraco.Core/CoreRuntime.cs
index b9d096bee7..b013c44e55 100644
--- a/src/Umbraco.Core/CoreRuntime.cs
+++ b/src/Umbraco.Core/CoreRuntime.cs
@@ -13,6 +13,7 @@ using Umbraco.Core.IO;
 using Umbraco.Core.Logging;
 using Umbraco.Core.Models.Rdbms;
 using Umbraco.Core.Persistence;
+using Umbraco.Core.Persistence.Mappers;
 using Umbraco.Core.Persistence.SqlSyntax;
 using Umbraco.Core.Plugins;
 using Umbraco.Core.Services;
@@ -199,11 +200,11 @@ namespace Umbraco.Core
             container.Register<ISqlSyntaxProvider, SqlCeSyntaxProvider>("SqlCeSyntaxProvider");
             container.Register<ISqlSyntaxProvider, SqlServerSyntaxProvider>("SqlServerSyntaxProvider");
 
-            // fixme explain
-            //// register persistence mappers - means the only place the collection can be modified
-            //// is in a runtime - afterwards it has been frozen and it is too late
-            //container.RegisterCollectionBuilder<MapperCollectionBuilder>()
-            //    .Add(f => f.GetInstance<PluginManager>().ResolveAssignedMapperTypes());
+            // register persistence mappers - required by database factory so needs to be done here
+            // means the only place the collection can be modified is in a runtime - afterwards it 
+            // has been frozen and it is too late
+            container.RegisterCollectionBuilder<MapperCollectionBuilder>()
+                .Add(f => f.GetInstance<PluginManager>().ResolveAssignedMapperTypes());
 
             // register database factory
             // will be initialized with syntax providers and a logger, and will try to configure
diff --git a/src/Umbraco.Core/DI/RepositoryCompositionRoot.cs b/src/Umbraco.Core/DI/RepositoryCompositionRoot.cs
index 68e7e71dd7..82b51f8305 100644
--- a/src/Umbraco.Core/DI/RepositoryCompositionRoot.cs
+++ b/src/Umbraco.Core/DI/RepositoryCompositionRoot.cs
@@ -101,12 +101,6 @@ namespace Umbraco.Core.DI
             // awkward but it works
             var serviceContainer = container as IServiceContainer;
             if (serviceContainer == null) throw new Exception("Container is not IServiceContainer.");
-
-            // fixme fixme fixme more - wtf?
-            // has moved to core runtime, why?
-            // register persistence mappers
-            serviceContainer.RegisterCollectionBuilder<MapperCollectionBuilder>()
-                .Add(f => f.GetInstance<PluginManager>().ResolveAssignedMapperTypes());
         }
     }
 }
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