Ionic Strength of 0.1M of HEPES?
The Ionic Strength is calculated as:

$$I=\dfrac{1}{2}\cdot\sum{c_iz_i^2},$$ where where $c_i$ is the molar concentration of ion i ($M,\,mol/L$), $z_i$ is the charge number of that ion, and the sum is taken over all ions in the solution.

This is your HEPES molecule ($C_8H_{18}N_2O_4S$):



Only one proton at $SO_3H$ group can dissociate, as far as I see. So in your solution you will have two different types of ions with charge equal to 1 (assuming that molecule will dissociates 100%, and than:

$I=0.5\cdot(0.1M\cdot1^2+0.1M\cdot(-1)^2)=0.1M$

So your calculations are wrong.

But also be aware that this molecule is Zwitterion! 
READ THIS Do zwitterions contribute to the ionic strength of a solution?
The resume of the articles:

"Hence, electrostatic effects due to zwitterions are important, even if the zwitterions do
not contribute directly to ionic strength."

My calculations are primitive. They are also far from the truth, but they show how the principle works for you.