Is there a problem with the following situation ?

> QupstrUnit <- NULL
> match.arg(arg = QupstrUnit, choices = c("mm", "m3", "m3/s", "l/s", "L/s"))
[1] "mm"

It's maybe exaggerated but we should prevent that a NULL value make a choice be default that it is not known by the user. I suggest to adopt one of these solutions:

• if(is.null(QupstrUnit)) stop("QupstrUnit should be one of \"", paste(choices, collapse = "\", \""), "\"")
• if(is.null(QupstrUnit)) warning("'QupstrUnit' has been redefined to \"mm\"")

Using tolower(QupstrUnit) fixed it and it allows to be flexible in the respect of the case of characters.

> match.arg(arg = tolower(NULL), choices = c("mm", "m3", "m3/s", "l/s"))
Error in match.arg(arg = tolower(NULL), choices = c("mm", "m3", "m3/s",  : 
  'arg' doit être un de “mm”, “m3”, “m3/s”, “l/s”
> match.arg(arg = NULL, choices = c("mm", "m3", "m3/s", "l/s"))
[1] "mm"

I wasn't shocked by the previous behavior, because that's how the match.arg() function usually works.

And it is a normal behavior in R to return the default value if the NULL value is used.