Error in if (num < 0) { : missing value where TRUE/FALSE needed

y<- as.integer(readline(prompt ="Enter a number: "))
factorial = 1
 if (y< 0){
    print("Error")
 } else if (y== 0) 
 {
     print("1")
 } else
 { 
     for(i in 1:y)  {
            factorial = factorial * i
                    }
      return(factorial)
  }

wondering why this is giving Error in if (y< 0) { : missing value where TRUE/FALSE needed

is it cause the first line has data type NA_integer?