diff --git a/util.go b/util.go
index 6ebb5d0..2a43348 100644
--- a/util.go
+++ b/util.go
@@ -8,7 +8,7 @@ import (
 // from http://stackoverflow.com/a/31832326
 var src = rand.NewSource(time.Now().UnixNano())
 
-const letterBytes = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
+const letterBytes = "abcdefghijklmnopqrstuvwxyz1234567890"
 const (
 	letterIdxBits = 6                    // 6 bits to represent a letter index
 	letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits