Calculate pH of a buffer solution with 1.0 M ammonia (NH3 ; Kb = 1.8 x 10-5) & 1.0 M ammonium chloride (NH4Cl)

For this, we can use the Henderson-Hasselbalch Equation, which states that:
pH = pKa + log([base]/[acid])

We can find the pKa of the solution by finding Ka from Kb
Kb*Ka = 10^-14
Ka = (10^-14)/(1.8*10^-5)
Ka = 5.56*10^-10
pKa = -logKa
pKa = 9.2553

Now, we can say:
pH = 9.2553 + log([base]/[acid])

We have 1 M ammonia, the base, and 1 M ammonium ions, the acid, so now we have:
pH = 9.2553 + log(1/1)
pH = 9.2553 + 0
pH = 9.2553

Because?