False positive on string variable assignment

[PiotrNestor]

Description

Analysing with the latest Docker version a simple HelloWorld contract.
The analysis reports some false positive for string variable assignments

How to Reproduce

// SPDX-License-Identifier: MIT
pragma solidity >=0.7.3;

contract HelloWorld {

   event UpdatedMessages(string oldStr, string newStr);
   string public message;

   constructor(string memory initMessage) {
      message = initMessage;
   }

   function update(string memory newMessage) public {
      string memory oldMsg = message;
      message = newMessage;
      emit UpdatedMessages(oldMsg, newMessage);
   }
}

In the report:

==== Integer Arithmetic Bugs ====
SWC ID: 101
Severity: High
Contract: HelloWorld
Function name: update(string)
PC address: 395
Estimated Gas Usage: 21046 - 86946
The arithmetic operator can underflow.
It is possible to cause an integer overflow or underflow in the arithmetic operation.
--------------------
In file: /tmp/HelloWorld.sol:28

string memory oldMsg = message

--------------------
Initial State:

Account: [CREATOR], balance: 0x0, nonce:0, storage:{}
Account: [ATTACKER], balance: 0x0, nonce:0, storage:{}

Transaction Sequence:

-->

Expected behavior

I suppose there no problem with the Solidity code

Environment

Mythril in Docker
Mythril version v0.23.17