Dapper - Null values are being inserted into MySql table

After hours of trying to figure out the problem, I am still stuck. Here is the problem:

I have a MySql query in my C# code:

var sql = "INSERT INTO MEMBERS(Salutation, FirstName, LastName) VALUES(?salutation, ?firstName, ?lastName)";

This query is in a method which accepts an ExpandoObject:

public async Task AddMember(ExpandoObject expando)
{
    var sql = "INSERT INTO MEMBERS(Salutation, FirstName, LastName) VALUES(?salutation, ?firstName, ?lastName)";

    // Convert the incoming ExpandoObject to Dapper's DynamicParameters
    // https://stackoverflow.com/questions/9481678/how-to-create-arguments-for-a-dapper-query-dynamically
    if (expando != null)
    {
        var dbArgs = new DynamicParameters();

        foreach (var param in expando)
        {
            dbArgs.Add(param.Key, param.Value);
        }

        await mySqlConnection.ExecuteAsync(sql, dbArgs);
    }
}

The query is executing (meaning that I am not getting an error, and a row is created in the Members table, and also the MemberId column value is generated), but Salutation, FirstName and LastName are null.

I checked the contents of the Expando. The input values are all there.

I tried various things.

• Using @ and : instead of ? in the query.
• Using the Expando directly in QueryAsync instead of converting it to DynamicParameters.
• Using an IDictionary as the input instead of the ExpandoObject and passing it directly to QueryAsync.
• Using QueryAsync instead of ExecuteAsync.

Nothing works. The 3 columns are null every time. Not sure what is wrong here.

Due to several reasons, I can't use a strong type like Member as the input to this method. I have to use an ExpandoObject.

• The MySql version I'm using is 5.7.13
• My Dapper version is 1.60.6
• The MySql.Data nuget package that I'm using is 8.0.17