为什么as.numic()函数在R中产生不同的结果

I am writing a short program in R that tells the user whether their input is numeric or not.
Here is my code:

numeric_check <- function() {
     input <- readline(prompt="Enter something: ")
     cat("Input: ", input, "\n")  
     cat("Input type: ", class(input), "\n")  
     check <- as.numeric(input)  
     cat("NA or number? ", check, "\n")  
     cat("Type:", class(check), "\n")
     if (is.na(check)) {
         print("Not numeric.")
     } else {
         print("Numeric.")
     }
 }

It works fine when I input a number.
However, when I input the string, "123", the "NA or number?" checkpoint prints out "NA", while the "Type" checkpoint prints out "numeric".
I checked the portion where I suspect might be the issue:

test <- as.numeric("123")
test

这表现为

123

instead of "NA" as it does in my program above.
Why does the program print out "NA" for as.numeric("123") and when I check it individually, it prints out the numeric type 123?
If someone can point to what I have missed, I would be very grateful. Thank you.