diff --git a/src/caosdb/common/models.py b/src/caosdb/common/models.py
index 4523b1f89d27e26dc32bb7f02485d9bd4acdff10..a5adf6af68b72663c0fe8cd6ab58c39cdcea5d27 100644
--- a/src/caosdb/common/models.py
+++ b/src/caosdb/common/models.py
@@ -2049,7 +2049,7 @@ class _Messages(dict):
             else:
                 raise TypeError(
                     "('type', 'code'), ('type'), or 'type' expected.")
-        if isinstance(key, _Messages._msg_key):
+        elif isinstance(key, _Messages._msg_key):
             type = key._type  # @ReservedAssignment
             code = key._code
         else:
@@ -2073,7 +2073,7 @@ class _Messages(dict):
             body = value
             description = None
         m = Message(type=type, code=code, description=description, body=body)
-        self.append(m)
+        dict.__setitem__(self, _Messages._msg_key(type, code), m)
 
     def __getitem__(self, key):
         if isinstance(key, tuple):
diff --git a/unittests/test_message.py b/unittests/test_message.py
index b209b0f5b851b929d42f0624cd2a9357755534f1..5e1003056c1b606a004b63bb7618e5e0474952bc 100644
--- a/unittests/test_message.py
+++ b/unittests/test_message.py
@@ -3,6 +3,8 @@
 # ** header v3.0
 # This file is a part of the CaosDB Project.
 #
+# Copyright (C) 2018 Research Group Biomedical Physics,
+# Max-Planck-Institute for Dynamics and Self-Organization Göttingen
 # Copyright (C) 2020 IndiScale GmbH <info@indiscale.com>
 # Copyright (C) 2020 Florian Spreckelsen <f.spreckelsen@indiscale.com>
 #
@@ -25,6 +27,61 @@ import caosdb as db
 from copy import deepcopy
 
 
+def test_messages_dict_behavior():
+    from caosdb.common.models import Message
+    from caosdb.common.models import _Messages
+
+    msgs = _Messages()
+
+    # create Message
+    msg = Message(
+        type="HelloWorld",
+        code=1,
+        description="Greeting the world",
+        body="Hello, world!")
+
+    # append it to the _Messages
+    assert repr(msg) == '<HelloWorld code="1" description="Greeting the world">Hello, world!</HelloWorld>\n'
+    msgs.append(msg)
+    assert len(msgs) == 1
+
+    # use _Messages as list of Message objects
+    for m in msgs:
+        assert isinstance(m, Message)
+
+    # remove it
+    msgs.remove(msg)
+    assert len(msgs) == 0
+
+    # ok append it again ...
+    msgs.append(msg)
+    assert len(msgs) == 1
+    # get it back via get(...) and the key tuple (type, code)
+    assert id(msgs.get("HelloWorld", 1)) == id(msg)
+
+    # delete Message via remove and the (type,code) tuple
+    msgs.remove("HelloWorld", 1)
+    assert msgs.get("HelloWorld", 1) is None
+    assert len(msgs) == 0
+
+    # short version of adding/setting/resetting a new Message
+    msgs["HelloWorld", 2] = "Greeting the world in German", "Hallo, Welt!"
+    assert len(msgs) == 1
+    assert msgs["HelloWorld", 2] == (
+        "Greeting the world in German", "Hallo, Welt!")
+
+    msgs["HelloWorld", 2] = "Greeting the world in German", "Huhu, Welt!"
+    assert len(msgs) == 1
+    assert msgs["HelloWorld", 2] == (
+        "Greeting the world in German", "Huhu, Welt!")
+    del msgs["HelloWorld", 2]
+    assert msgs.get("HelloWorld", 2) is None
+
+    # this Message has no code and no description (make easy things easy...)
+    msgs["HelloWorld"] = "Hello!"
+    assert msgs["HelloWorld"] == "Hello!"
+
+
 def test_deepcopy():
     """Test whether deepcopy of _Messages objects doesn't mess up
     contained Messages objects.