diff --git a/index.js b/index.js
index b06ac2b..6c2d483 100644
--- a/index.js
+++ b/index.js
@@ -22,35 +22,49 @@
     assert(tokenChars.length < 256);
   }
 
-  function buildGenerator(options) {
-    assert(!options || typeof(options) == 'object');
-    options = options || {};
-    options.chars = options.chars || defaults.chars;
-    options.source = options.source || defaults.source;
-
-    // Allowed characters
-    switch( options.chars ) {
+  function getChars (chars) {
+    var result;
+    switch( chars ) {
       case 'default':
-        options.chars = alphaNumeric;
+        result = alphaNumeric;
         break;
       case 'a-z':
       case 'alpha':
-        options.chars = alphaLower;
+        result = alphaLower;
         break;
       case 'A-Z':
       case 'ALPHA':
-        options.chars = alphaUpper;
+        result = alphaUpper;
         break;
       case '0-9':
       case 'numeric':
-        options.chars = numeric;
+        result = numeric;
         break;
       case 'base32':
-        options.chars = alphaUpper + "234567";
+        result = alphaUpper + "234567";
         break;
       default:
         // use the characters as is
     }
+    return result;
+  }
+
+  function buildGenerator(options) {
+    assert(!options || typeof(options) == 'object');
+    options = options || {};
+    options.chars = options.chars || defaults.chars;
+    options.source = options.source || defaults.source;
+
+    // Allowed characters
+    if (Array.isArray(options.chars)) {
+      var charsSet = '';
+      options.chars.forEach(function(item) {
+        charsSet += getChars(item);
+      });
+      options.chars = charsSet;
+    } else {
+      options.chars = getChars(options.chars);
+    }
     validateTokenChars(options.chars);
 
     // Source of randomness:
diff --git a/test/test.js b/test/test.js
index a621710..c0c14d7 100644
--- a/test/test.js
+++ b/test/test.js
@@ -49,6 +49,8 @@ var tokenCharsToTest = [
   {chars: 'alpha', regex: /^[a-z]{16}$/ },
   {chars: 'ALPHA', regex: /^[A-Z]{16}$/ },
   {chars: 'numeric', regex: /^[0-9]{16}$/ },
+  {chars: ['ALPHA', 'numeric'], regex: /^[a-z0-9]{16}$/},
+  {chars: ['alpha', 'ALPHA', 'numeric'], regex: /^[a-zA-Z0-9]{16}$/},
 ];
 
 _.each(randSourceToTest, function(randSourceTest) {