diff --git a/routes/avatars.js b/routes/avatars.js
index c6cd0f1..839f794 100644
--- a/routes/avatars.js
+++ b/routes/avatars.js
@@ -1,41 +1,46 @@
 var express = require('express');
 var router = express.Router();
 var skins = require('../skins');
-var fs = require('fs')
+var fs = require('fs');
+
+var valid_uuid = /^[0-9a-f]{32}$/;
 
 /* GET home page. */
 router.get('/:uuid/:size?', function(req, res) {
   //res.render('index', { title: 'Express' });
   //res.send("uuid is set to " + req.param("uuid"));
   //console.log(req.param('size'))
-  var uuid = req.param('uuid')
-  var size = req.param('size')
-  if (size == null) {
-    size = 180;
-  }
-  var filename = 'skins/' + uuid + ".png";
-  if (fs.existsSync(filename)) {
-    skins.extract_face(filename, size, function() {
-      skins.extract_face(filename, size, function(data) {
-        res.writeHead(200, {'Content-Type': 'image/png'});
-        res.end(data);
-      });
-    });
-  } else {
-    skins.get_profile(uuid, function(profile) {
-    	var skinurl = skins.skin_url(profile);
-      if (skinurl) {
-        skins.skin_file(skinurl, filename, function() {
-          skins.extract_face(filename, size, function(data) {
-            res.writeHead(200, {'Content-Type': 'image/png'});
-            res.end(data);
-          });
+  var uuid = req.param('uuid');
+  var size = req.param('size') || 180;
+  console.log(uuid);
+  if (valid_uuid.test(uuid)) {
+    var filename = 'skins/' + uuid + ".png";
+    if (fs.existsSync(filename)) {
+      skins.extract_face(filename, size, function() {
+        skins.extract_face(filename, size, function(data) {
+          res.writeHead(200, {'Content-Type': 'image/png'});
+          res.end(data);
         });
-      } else {
-    		res.status(404)        // HTTP status 404: NotFound
-        .send('404 Not found')
-      }
-    });
+      });
+    } else {
+      skins.get_profile(uuid, function(profile) {
+        var skinurl = skins.skin_url(profile);
+        if (skinurl) {
+          skins.skin_file(skinurl, filename, function() {
+            skins.extract_face(filename, size, function(data) {
+              res.writeHead(200, {'Content-Type': 'image/png'});
+              res.end(data);
+            });
+          });
+        } else {
+          res.status(404)        // HTTP status 404: NotFound
+          .send('404 Not found');
+        }
+      });
+    }
+  } else {
+    res.status(422) // "Unprocessable Entity", valid request, but semantically erroneous: https://tools.ietf.org/html/rfc4918#page-78
+    .send("422 Invlid UUID");
   }
 });