unexpected EOF

When trying to decode a ppm I get that error, however when I write the data to a .ppm file my image reader reads the image fine.
```go
f, _ := os.Create(fileName)
f.Write(b.Bytes())
f.Close()

reader := bytes.NewReader(b.Bytes())
_, err := pnm.Decode(reader)

if err != nil {
	fmt.Println(err)
	return
}
```
any ideas as to why I might be getting this error?