diff --git a/lib/querystring.js b/lib/querystring.js
index 7999ac33f76..53752519290 100644
--- a/lib/querystring.js
+++ b/lib/querystring.js
@@ -310,8 +310,8 @@ QueryString.parse = QueryString.decode = function(qs, sep, eq, options) {
           encodeCheck = 1;
         } else if (encodeCheck > 0 &&
                    ((code >= 48/*0*/ && code <= 57/*9*/) ||
-                    (code >= 65/*A*/ && code <= 70/*Z*/) ||
-                    (code >= 97/*a*/ && code <= 102/*z*/))) {
+                    (code >= 65/*A*/ && code <= 70/*F*/) ||
+                    (code >= 97/*a*/ && code <= 102/*f*/))) {
           if (++encodeCheck === 3)
             valEncoded = true;
         } else {
@@ -341,8 +341,8 @@ QueryString.parse = QueryString.decode = function(qs, sep, eq, options) {
             encodeCheck = 1;
           } else if (encodeCheck > 0 &&
                      ((code >= 48/*0*/ && code <= 57/*9*/) ||
-                      (code >= 65/*A*/ && code <= 70/*Z*/) ||
-                      (code >= 97/*a*/ && code <= 102/*z*/))) {
+                      (code >= 65/*A*/ && code <= 70/*F*/) ||
+                      (code >= 97/*a*/ && code <= 102/*f*/))) {
             if (++encodeCheck === 3)
               keyEncoded = true;
           } else {