Maybe the first reviewer could not understand the derivation or the paper (perhaps it is outside his/her field)? If they have found something wrong, I believe they would (and should) have pointed it out! If you are comfortable with the results and as you said "double checked them", you should be fine. You can address his comment by stating that the results have been rechecked and validated (if possible) - as my advisor says, a vague question requires a general answer!
I will play the devil advocates for a second. I understand that the second reviewer is happy with the results (but, that does not assure us (the readers), that the results are fine). For all we know, s/he did not even go through them! Or maybe they did, but could not see the point of the first reviewer (if any). The first reviewer's comment is vague and to be it to rest, perhaps, you can ask a colleague of yours (who knows your field) to go over them again?