How to test method with reflect?

I have a method Sync() that overrides Config field's values that are set in the environment. The environment variable names are derived from config fields by underscoring, and uppercasing the name. E.g. AppName will have a corresponding environment variable APP_NAME

Please help me to test the following cases. There are complex things like http://ift.tt/29rYFva:

Set assigns x to the value v. It panics if CanSet returns false. As in Go, x's value must be assignable to v's type.

So I don't know how to test this case?

import (
    "encoding/json"
    "errors"
    "fmt"
     "os"
    "path/filepath"
    "reflect"
    "strconv"
    "strings"
    "http://ift.tt/1dBcdq3"
    "http://ift.tt/29rYksq"
    "gopkg.in/yaml.v2"
)

type Config struct {
    AppName   string 
    BaseURL   string 
    Port      int    
    Verbose   bool   
    StaticDir string 
    ViewsDir  string 
}

func (c *Config) Sync() error {
    cfg := reflect.ValueOf(c).Elem()
    cTyp := cfg.Type()

    for k := range make([]struct{}, cTyp.NumField()) {
        field := cTyp.Field(k)

        cm := getEnvName(field.Name)
        env := os.Getenv(cm)
        if env == "" {
            continue
        }
        switch field.Type.Kind() {
        case reflect.String:
            cfg.FieldByName(field.Name).SetString(env)
        case reflect.Int:
            v, err := strconv.Atoi(env)
            if err != nil {
                return fmt.Errorf("loading config field %s %v", field.Name, err)
            }
            cfg.FieldByName(field.Name).Set(reflect.ValueOf(v))
        case reflect.Bool:
            b, err := strconv.ParseBool(env)
            if err != nil {
                return fmt.Errorf("loading config field %s %v", field.Name, err)
            }
            cfg.FieldByName(field.Name).SetBool(b)
        }

    }
    return nil
}