I get the “TypeError: RepeatedCompositeFieldContainer object does not support item assignment” when accessing the Protobuf map in Python

Proto definition:

syntax = "proto3";
package test;
  
message TestMap {

    map<string, string> myMap = 1;
}

Accessing from Python:

test = test.TestMap()
test.myMap["key"] = "value"

The error message I get is:

  File "PythonTest.py", line 19, in testfunction
    test.myMap["key"] = "value"
    │    └ <google.protobuf.internal.python_message._FieldProperty object at 0x7f9866137820>
    └

  File "..../python-3.11/lib/python3.11/site-packages/google/protobuf/internal/containers.py", line 325, in __setitem__
    raise TypeError(

TypeError: RepeatedCompositeFieldContainer object does not support item assignment

Protobuf version: 4.25.1

What could be the issue? Thanks