How to make custom variants of fmt.Printf() and fmt.Sprintf() etc.?

Query string: ?invalid-param

values := r.URL.Query()
for k, v := range values {
    switch k {
    case "valid-query-param":
        // some code
    default:
        fmt.Printf("%s %Tn", k, k)
        Customf("Invalid query param: %s", k)
        return
    }
}

func Customf(s string, args ...any){
    fmt.Printf(s, args)
}

Output

invalid-param string
Invalid query param: [invalid-param]

In the output first it says that the query param k is a string, but in the second line it looks like it’s an []string? Why is that?