diff --git a/bin/twhois b/bin/twhois
index b142510..dcc6735 100755
--- a/bin/twhois
+++ b/bin/twhois
@@ -4,25 +4,32 @@ require 'twhois'
 
 if ARGV.size > 0
   ARGV.each do |username|
-    user = Twhois.lookup(username)
-    if user
-      puts "@#{user.screen_name}:"
-      puts "  Name: #{user.name}"
-      puts "  URL: #{user.url}" if user.url
-      puts "  Location: #{user.location}" if user.location
-      puts "  Description: #{user.description}" if user.description
-      puts "  Followers: #{user.followers_count}"
-      puts "  Following: #{user.friends_count}"
-      puts "  Tweets: #{user.statuses_count}"
-      puts "  Last Tweet (#{user.status['created_at']}):"
-      puts "    #{user.status['text']}"
-      puts "  Timezone: #{user.time_zone}" if user.time_zone
-      puts "  Joined On: #{user.created_at}"
-      puts "  Profile Picture: #{user.profile_image_url}"
-      puts "  Private Account: #{user.protected ? "Yes" : "No"}"
-    else
-      puts "@#{username}:"
-      puts "  Not Found"
+    begin
+      user = Twhois.lookup(username)
+      if user
+        puts "@#{user.screen_name}:"
+        puts "  Name: #{user.name}"
+        puts "  URL: #{user.url}" if user.respond_to?(:url)
+        puts "  Profile: http://twitter.com/#{user.screen_name}"
+        puts "  Location: #{user.location}" if user.respond_to?(:location)
+        puts "  Description: #{user.description}" if user.respond_to?(:description)
+        puts "  Followers: #{user.followers_count}"
+        puts "  Following: #{user.friends_count}"
+        puts "  Tweets: #{user.statuses_count}"
+        if user.respond_to?(:status)
+          puts "  Last Tweet (#{user.status['created_at']}):"
+          puts "    #{user.status['text']}"
+        end
+        puts "  Timezone: #{user.time_zone}" if user.respond_to?(:time_zone)
+        puts "  Joined On: #{user.created_at}"
+        puts "  Profile Picture: #{user.profile_image_url}"
+        puts "  Private Account: #{user.protected ? "Yes" : "No"}"
+      else
+        puts "@#{username}:"
+        puts "  Not Found"
+      end
+    rescue Twhois::InvalidUsername => e
+      puts "\"#{username}\" is not a valid Twitter username"
     end
   end
 else
diff --git a/lib/twhois.rb b/lib/twhois.rb
index 5e4db43..c6007b0 100644
--- a/lib/twhois.rb
+++ b/lib/twhois.rb
@@ -12,12 +12,20 @@ module Twhois
   LOOKUP_HOST = "api.twitter.com"
   LOOKUP_PATH = "/1/users/show.json?screen_name="
   
+  class InvalidUsername < StandardError; end
+  
   # Lookup a Twitter user by their username.
   def self.lookup(username)
+    raise InvalidUsername, "Username is invalid" unless valid_username?(username)
     res = Net::HTTP.start(LOOKUP_HOST) { |http| http.get(LOOKUP_PATH + username) }
     if res.code == '200'
       User.new(JSON.parse(res.body))
     end
   end
   
+  def self.valid_username?(username)
+    return false if username.match(/^[a-zA-Z0-9_]{1,15}$/).nil?
+    return true
+  end
+  
 end
diff --git a/spec/twhois_spec.rb b/spec/twhois_spec.rb
index 5a562d6..a456f19 100644
--- a/spec/twhois_spec.rb
+++ b/spec/twhois_spec.rb
@@ -18,8 +18,18 @@ describe Twhois do
   end
   
   it "should return error on unknown user" do
-    user = Twhois.lookup('jimehoawhefoahelfhasdf')
+    user = Twhois.lookup('akjsdfkjasdfasd')
     user.should be_nil
   end
   
+  it "should raise an exception on invalid usernames" do
+    lambda { # invalid characters
+      user = Twhois.lookup("abc/damn")
+    }.should raise_error(Twhois::InvalidUsername)
+    
+    lambda { # longer than 15 characters
+      user = Twhois.lookup("abcasdjfakajsdfasdfasdfa")
+    }.should raise_error(Twhois::InvalidUsername)
+  end
+  
 end
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