diff --git a/ln.json/ln.json.csproj b/ln.json/ln.json.csproj
index 4c37b94..c0b6510 100644
--- a/ln.json/ln.json.csproj
+++ b/ln.json/ln.json.csproj
@@ -9,7 +9,7 @@
         <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
         <AssemblyVersion>0.1.0.0</AssemblyVersion>
         <FileVersion>0.1.0.0</FileVersion>
-        <PackageVersion>1.2.2</PackageVersion>
+        <PackageVersion>1.2.3</PackageVersion>
         <TargetFrameworks>net5.0;net6.0</TargetFrameworks>
     </PropertyGroup>
 
diff --git a/ln.json/mapping/JSONMapper.cs b/ln.json/mapping/JSONMapper.cs
index db06040..e42ea04 100644
--- a/ln.json/mapping/JSONMapper.cs
+++ b/ln.json/mapping/JSONMapper.cs
@@ -66,7 +66,13 @@ namespace ln.json.mapping
         Dictionary<Type, MappingFactory> mappingFactories = new Dictionary<Type, MappingFactory>();
         public virtual void AddMappingFactory(Type targetType, MappingFactory mappingFactory) => mappingFactories.Add(targetType, mappingFactory);
 
-        public virtual bool GetOrBuildMapping(Type nativeType, out JSONMapping mapping) => TryGetMapping(nativeType, out mapping) || TryBuildRememberedMapping(nativeType, out mapping);
+        public virtual bool GetOrBuildMapping(Type nativeType, out JSONMapping mapping)
+        {
+            lock (mappings)
+            {
+                return TryGetMapping(nativeType, out mapping) || TryBuildRememberedMapping(nativeType, out mapping);
+            }
+        }
 
         public virtual bool TryBuildRememberedMapping(Type nativeType,out JSONMapping mapping)
         {