Function also applies to NA values

Question

I'm trying to apply a fuzzify a Terra SpatRaster object using a created Fuzzy membership function with FuzzyR package using Terra::app, but the function is also aplied to NA values. How can I avoid this problem?

FRea_0C_rast <- rasterize(FRea_0C, area_rast, "FRea_value")

FRea_0C_rast_funfuzz <- genmf("trimf", c(0, 0, 7))
FRea_0C_rast_fuzz <- app(FRea_0C_rast, FRea_0C_rast_funfuzz)

I've tried to include arguments such as na.rm but they don't work on the app function.