"No implicit conversion between type A and type B" error when using ternary operator

Question

Why am I not allowed to do this:

private Base Property => m_IsAbsolute ? new A() : new B();

Compiler complains about 'No implicit conversion between A and B

I can fix that by casting B to Base, but why on earth would I need to do that?

While I am allowed to do this:

private Base Property2
{
    get
    {
        if (m_IsAbsolute)
        {
            return new A();
        }
        return new B();
    }
}

For reference the classes used in above example:

class Base
{
}

class A : Base
{
}

class B : Base
{        
}