Errors and omissions

Technically speaking, the missing value created by NA() is a type of error. You can test for errors using ISERROR(). Similar to the other IS*() functions that you've seen, it takes a cell address as its input and returns TRUE if that cell contains an error and FALSE otherwise.

Treating missing values as errors is often undesirable since real-world data naturally contains missing values. That's why there is also ISERR() that returns TRUE for all error types except missing values.