NSO and cost basis filing

I had exercised Non-qualified stock options using a same day option.  Taxes were withheld at the time of the transaction on the difference between the strike price and sale price and the income was reported on my W2 identified as code "V".  I received a notification from the IRS stating I under reported income and is referring to an amount that appears to be the overall value of the shares not the actual gain I realized.  I am [fairly] certain the IRS is incorrect.  Done lots of research and everything I find steers me back to that I am correct.  However, any feedback from the forum?