diff --git a/Pilz.Net/Api/ApiServer.cs b/Pilz.Net/Api/ApiServer.cs
index 71f15a2..263188d 100644
--- a/Pilz.Net/Api/ApiServer.cs
+++ b/Pilz.Net/Api/ApiServer.cs
@@ -76,14 +76,19 @@ public class ApiServer : IApiServer
     {
         var curThread = ThreadedDataManager ? Thread.CurrentThread : null;
         var threadHolder = new ThreadHolder(curThread);
+        IDataManager manager;
 
-        if (managers.TryGetValue(threadHolder, out var mgr))
-            return mgr;
+        lock (managers)
+        {
+            if (managers.TryGetValue(threadHolder, out var mgr))
+                return mgr;
 
-        if (OnGetNewDataManager?.Invoke(this, EventArgs.Empty) is not IDataManager manager)
-            throw new MissingDataManagerException();
-        
-        managers.Add(threadHolder, manager);
+            if (OnGetNewDataManager?.Invoke(this, EventArgs.Empty) is not IDataManager managerr)
+                throw new MissingDataManagerException();
+            manager = managerr;
+
+            managers.Add(threadHolder, manager);
+        }
 
         return manager;
     }