diff --git a/test/test_language.rb b/test/test_language.rb
index 1ad47e33..aeb73127 100644
--- a/test/test_language.rb
+++ b/test/test_language.rb
@@ -371,4 +371,19 @@ class TestLanguage < Test::Unit::TestCase
     message << missing.map { |language| sprintf("%-#{width}s %s", language.name, language.tm_scope) }.sort.join("\n")
     assert missing.empty?, message
   end
+
+  def test_all_languages_have_a_valid_ace_mode
+    ace_github_modes = JSON.parse `curl https://api.github.com/repos/ajaxorg/ace/contents/lib/ace/mode`
+    existing_ace_modes = ace_github_modes.map do |ace_github_mode|
+      File.basename(ace_github_mode["name"], ".js") if ace_github_mode["name"]  !~ /_highlight_rules|_test|_worker/
+    end.compact.uniq.sort
+
+    missing = Language.all.reject { |language| language.ace_mode == "none" || existing_ace_modes.include?(language.ace_mode) }
+    message = "The following languages do not have an Ace mode listed in languages.yml. Please add an Ace mode for all new languages.\n"
+    message << "If no Ace mode exists for a language, mark the language with `ace_mode: none` in lib/linguist/languages.yml.\n"
+
+    width = missing.map { |language| language.name.length }.max
+    message << missing.map { |language| sprintf("%-#{width}s %s", language.name, language.ace_mode) }.sort.join("\n")
+    assert missing.empty?, message
+  end
 end