Dapper must declare the scalar variable

I am experiencing issues when trying to used a paramaterized query in Dapper. I have found a number of other users with similar issues but have not been able to resolve the issue.

The code

    public User GetUser(int employeeId)
    {
        var args = new
        {
            EmployeeId = employeeId
        };

        const string sql = @"
                    select 
                        first_name 'FirstName', 
                        last_name 'LastName'
                    from 
                        users 
                    where 
                        employee_id = @EmployeeId 
                ";

        using (var con = MakeConnection())
        {
            var r = con.Query<User>(sql, args);
            return r.FirstOrDefault();
        }
    }

The error

A first chance exception of type 'System.Data.Odbc.OdbcException' occurred in System.Data.dll

Additional information: ERROR [42000] [Microsoft][ODBC SQL Server Driver][SQL Server]Must declare the scalar variable "@EmployeeId".

I have also tried using DynamicParameters and passing that instead but that is not working either

var p = new DynamicParameters();
p.Add("@EmployeeId", employeeId); // I have also tried without the @
//...
var r = con.Query<User>(sql,p);