MethodInfo always null when geting Type from instance

Can someone explain me, why here I get the correct MethodInfo:

MethodInfo mi = typeof(ContextTimerMode).GetMethod(_vo.Phase, BindingFlags.Instance | BindingFlags.NonPublic);

if (mi != null)
{
    mi.Invoke(this, new object[] { btns, vo });
}

While trying to get it directly from instance returns always null?:

MethodInfo mi = (this as ContextTimerMode).GetType().GetMethod(_vo.Phase, BindingFlags.Instance | BindingFlags.NonPublic);

// mi always null
if (mi != null)
{
    mi.Invoke(this, new object[] { btns, vo });
}

The code above is from ContextTimerMode.

this is a class that has ContextTimerMode as base class;

Actually I couldn't find a reason why it returns a null for sometime as I was using second method and I was debugging and making sure _vo.Phase string has correct name of method so I tried first method and it worked out.

Also when debugging this was showing that this is instance not of ContextTimerMode but a type that has a base of ContextTimerMode - so that is why I tried with (this as ContextTimerMode).GetType()...