Union with different dataTypes Linq-to-entities

I'm trying to return a grid! with headers and values below. I'm trying to do this!

void Main()
{
    var ctx = dbContext;
    (from tQuestions in ctx.Questions
        select new 
        {
            a1_1 = (Object)tQuestions.HeaderCreationDate //varchar(max)   (sometning like "Creation Date")
        }
    ).Union<Object>(from tAnswers in ctx.Answers
        select new{
        a1_1 = (Object)tAnswers.CreationDate //DateTime (something like "2018-05-24 00:00:00.000")
        }
    ).Dump();
}

And I have: "Conversion failed when converting date and / or time from character string."