Theoretical infinite voltage here! Where's the problem?
Your formula is wrong. 

The correct voltage across the external resistor is 

$$
\begin{aligned}
&amp;\frac{V_\text{ext}}{V_\text{int}} = \frac{R_\text{ext}}{R_\text{int}+R_\text{ext}}\\\\
\end{aligned}
$$

So the voltage Vint approaches Vext when Rext approaches infinity.