Why does enthalpy not change in isothermal gas expansion?

[Luscinia]

Homework Statement

The internal energy of a perfect gas does not change when the gas undergoes isothermal expansion. What is the change in enthalpy?

Homework Equations

H=U+PV

The Attempt at a Solution

The answer is 0

Since U, the internal energy, doesn't change, I assumed that the equation would be ΔH=PΔV. Why would PΔV equal 0 if there is an expansion (ΔV is not 0)? I'm guessing that I'm not seeing something here since this seemed to be a rather simple question.

 

[AGNuke]

The enthalpy is actually $\Delta H = \Delta U + \Delta (PV)$. I don't think that PV will change.