Invalid indirect of type func (int) string

I'm getting stucked with the following error:

./main.go:76: invalid indirect of Fizzbuzz (type func(int) string)

I understand that the Fizzbuzz function does not satisfy the writeString. My intuition is telling me that this is probably because I should be using an interface to Fizzbuzz? Can someone please give me some direction on how to execute this? What can I do to make this code Go idiomatic?

// -------------------------------INPUT--------------------------------------
// Your program should read an input file (provided on the command line),
// which contains multiple newline separated lines.
// Each line will contain 3 numbers which are space delimited.
// The first number is first number to divide by ('A' in this example),
// the second number is the second number to divide by ('B' in this example)
// and the third number is where you should count till ('N' in this example).
// You may assume that the input file is formatted correctly and the
// numbers are valid positive integers. E.g.
// 3 5 10
// 2 7 15
// -------------------------------OUTPUT------------------------------------
// Print out the series 1 through N replacing numbers divisible by 'A' by F,
// numbers divisible by 'B' by B and numbers divisible by both as 'FB'.
// Since the input file contains multiple sets of values, your output will
// print out one line per set. Ensure that there are no trailing empty spaces
// on each line you print. E.g.
// 1 2 F 4 B F 7 8 F B
// 1 F 3 F 5 F B F 9 F 11 F 13 FB 15
// ---------------------------PROPOSED SOLUTION-----------------------------
package main
import (
"bufio"
"fmt"
"log"
"os"
)
func Fizzbuzz(N int) (output string) {
var (
A = N%3 == 0
B = N%5 == 0
)
switch {
case A && B:
output = "FB"
case A:
output = "F"
case B:
output = "B"
default:
output = fmt.Sprintf("%v", N)
}
return
}
func openFile(name string) *os.File {
file, err := os.Open(name)
if err != nil {
log.Fatalf("failed opening %s for writing: %s", name, err)
}
return file
}
func Readln(r *bufio.Reader) {
line, prefix, err := r.ReadLine()
if err != nil {
log.Fatalf("failed reading a line: %v", err)
}
if prefix {
log.Printf("Line is too big for buffer, only first %d bytes returned", len(line))
}
}
func WriteString(w *bufio.Writer) {
if n, err := w.WriteString(*Fizzbuzz); err != nil {
log.Fatalf("failed writing string: %s", err)
} else {
log.Printf("Wrote string in %d bytes", n)
}
}
func main() {
file := openFile(os.Args[1])
defer file.Close()
fi := bufio.NewReader(file)
Readln(fi)
fo := bufio.NewWriter(file)
defer fo.Flush()
WriteString(fo)
}

Go-Playground

* as a unary operator is used to dereference (or "indirect") a pointer. Fizzbuzz is a function, not a pointer. That is why the compiler says:

>Invalid indirect of type func (int) string

What you really want to do is call the function: Fizzbuzz()

So line:

if fizzbuzz, err := w.WriteString(*Fizzbuzz); err != nil {

should be:

if fizzbuzz, err := w.WriteString(Fizzbuzz()); err != nil{

It is not very idiomatic to call the first return of writestring something like fizzbuzz. Normally we name it "n".

if n, err := w.WriteString(Fizzbuzz()); err != nil{