diff --git a/dlt/common/plugins/plugins.py b/dlt/common/plugins/plugins.py
index af1d187694..784812c8b7 100644
--- a/dlt/common/plugins/plugins.py
+++ b/dlt/common/plugins/plugins.py
@@ -12,13 +12,14 @@
 from dlt.common.configuration.specs.base_configuration import BaseConfiguration
 import multiprocessing as mp
 from functools import wraps
+import threading
 
 
 def on_main_process(f: TFun) -> TFun:
     @wraps(f)
     def _wrap(self: "PluginsContext", *args: Any, **kwargs: Any) -> Any:
         # send message to shared queue if this is not the main instance
-        if not self._main:
+        if not self._main or not threading.main_thread():
             self._queue.put((f.__name__, args, kwargs))
             return None
         return f(self, *args, **kwargs)
diff --git a/tests/pipeline/test_schema_contracts.py b/tests/pipeline/test_schema_contracts.py
index bf2dc1ba31..62a75c8a9c 100644
--- a/tests/pipeline/test_schema_contracts.py
+++ b/tests/pipeline/test_schema_contracts.py
@@ -186,7 +186,7 @@ def first_row(self) -> Dict[str, Any]:
         data_item = (
             call.data_item
             if not (isinstance(call.data_item, Iterable) and not isinstance(call.data_item, dict))
-            else call.data_item[0] # type: ignore
+            else call.data_item[0]  # type: ignore
         )
         # arrow tables
         if hasattr(data_item, "num_rows"):