Nutrafin Mini Master Test Kit and Salt Water

[Oscarr19]

I'm having problems with my test kit. I know it's not the best but I'm only working with it for the basics and plan to upgrade when I get corals. Anyway, I'm getting an ammonia reading only from saltwater. I'm currently using tap but I'm switching to RO/DI soon. My tap tests for 0 across the board but my tank tests for 0.6 ammonia all the time and so does freshly made salt water. Could it be that the test kit shows an ammonia reading with saltwater only even if there isn't a trace? I'm using fluval sea salt by the way.