Test kits faulty????!!!!

[rvm]

I just went out and bought a new test kit as I'm running low on my current one. I bought the Hagen mini master kit (same as the master, but only tests ammonia, nitrite, nitrate and Ph). I tested the tank and all was going well until I got to the Nitrate test. It read at 0!!! This would be fine except for my other test kit (Interpet Easytest) always gives me readings of 50ish.
I had a couple of nitrate tests left in the Interpet kit, so I tested the water again with it, and sure enough, it read 50!! So I thought maybe I'd done the Hagen one wrong so I re read the instructions and followed them to the letter, and once again it read 0!!!
One of my kits has to be wrong, but which one is it???

 

[luxum]

The one that says zero nitrate is likely wrong unless you have a whole lot of plants growing strong in high light. This is very common in test kits, sometimes they expire while sitting on the shelf waiting to be sold, sometimes they are just defective. I'd return the whole master kit to the store and get a new one, see if you can find an expiration date on the packaging.

 

[rvm]

it says expiry 31/12/2007, so dunno if it's that. I do have a lot of plants in quite high light, but I've never had my nitrate below 25 before, so i reckon it's the hagen thats wrong as well. I'll take it back tomorrow!!!