diff --git a/testing/test_workermanage.py b/testing/test_workermanage.py
index fae06011..b19c524a 100644
--- a/testing/test_workermanage.py
+++ b/testing/test_workermanage.py
@@ -3,9 +3,12 @@
 import pytest
 import shutil
 import textwrap
+import warnings
 from pathlib import Path
+from util import generate_warning
 from xdist import workermanage
-from xdist.workermanage import HostRSync, NodeManager
+from xdist.remote import serialize_warning_message
+from xdist.workermanage import HostRSync, NodeManager, unserialize_warning_message
 
 pytest_plugins = "pytester"
 
@@ -326,3 +329,80 @@ def test_one():
         )
         (rep,) = reprec.getreports("pytest_runtest_logreport")
         assert rep.passed
+
+
+class MyWarning(UserWarning):
+    pass
+
+
+@pytest.mark.parametrize(
+    "w_cls",
+    [
+        UserWarning,
+        MyWarning,
+        "Imported",
+        pytest.param(
+            "Nested",
+            marks=pytest.mark.xfail(reason="Nested warning classes are not supported."),
+        ),
+    ],
+)
+def test_unserialize_warning_msg(w_cls):
+    """Test that warning serialization process works well"""
+
+    # Create a test warning message
+    with pytest.warns(UserWarning) as w:
+        if not isinstance(w_cls, str):
+            warnings.warn("hello", w_cls)
+        elif w_cls == "Imported":
+            generate_warning()
+        elif w_cls == "Nested":
+            # dynamic creation
+            class MyWarning2(UserWarning):
+                pass
+
+            warnings.warn("hello", MyWarning2)
+
+    # Unpack
+    assert len(w) == 1
+    w_msg = w[0]
+
+    # Serialize and deserialize
+    data = serialize_warning_message(w_msg)
+    w_msg2 = unserialize_warning_message(data)
+
+    # Compare the two objects
+    all_keys = set(vars(w_msg).keys()).union(set(vars(w_msg2).keys()))
+    for k in all_keys:
+        v1 = getattr(w_msg, k)
+        v2 = getattr(w_msg2, k)
+        if k == "message":
+            assert type(v1) == type(v2)
+            assert v1.args == v2.args
+        else:
+            assert v1 == v2
+
+
+class MyWarningUnknown(UserWarning):
+    # Changing the __module__ attribute is only safe if class can be imported
+    # from there
+    __module__ = "unknown"
+
+
+def test_warning_serialization_tweaked_module():
+    """Test for GH#404"""
+
+    # Create a test warning message
+    with pytest.warns(UserWarning) as w:
+        warnings.warn("hello", MyWarningUnknown)
+
+    # Unpack
+    assert len(w) == 1
+    w_msg = w[0]
+
+    # Serialize and deserialize
+    data = serialize_warning_message(w_msg)
+
+    # __module__ cannot be found!
+    with pytest.raises(ModuleNotFoundError):
+        unserialize_warning_message(data)
diff --git a/testing/util.py b/testing/util.py
new file mode 100644
index 00000000..c7bcc552
--- /dev/null
+++ b/testing/util.py
@@ -0,0 +1,9 @@
+import warnings
+
+
+class MyWarning2(UserWarning):
+    pass
+
+
+def generate_warning():
+    warnings.warn(MyWarning2("hello"))