diff --git a/main.go b/main.go
index 7ad17bb..53ec649 100644
--- a/main.go
+++ b/main.go
@@ -2,15 +2,16 @@ package gocaptcha
 
 import (
   "crypto/rand"
+  "math/big"
   "encoding/base64"
   "image"
   "image/color"
   "image/draw"
   "image/png"
   "io/ioutil"
-  "math/big"
   "strings"
   "bytes"
+  "log"
 
   "golang.org/x/image/font"
   "golang.org/x/image/font/sfnt"
@@ -90,10 +91,14 @@ func VerifyCaptcha(input, captchaText string) bool {
 }
 
 func genTxt(charset string, length int) string {
+  runes := []rune(charset)
   text := make([]rune, length)
   for i := range text {
-    randomIndex, _ := rand.Int(rand.Reader, big.NewInt(int64(len(charset))))
-    text[i] = rune(charset[randomIndex.Int64()])
+    randomIndex, err := rand.Int(rand.Reader, big.NewInt(int64(len(runes))))
+    if err != nil {
+      log.Fatal(err)
+    }
+    text[i] = runes[randomIndex.Int64()]
   }
   return string(text)
 }