diff --git a/src/workflow/ArcanistSetConfigWorkflow.php b/src/workflow/ArcanistSetConfigWorkflow.php
index 9d9b099d..7bad38c1 100644
--- a/src/workflow/ArcanistSetConfigWorkflow.php
+++ b/src/workflow/ArcanistSetConfigWorkflow.php
@@ -1,131 +1,141 @@
 <?php
 
 /**
  * Write configuration settings.
  */
 final class ArcanistSetConfigWorkflow extends ArcanistWorkflow {
 
   public function getWorkflowName() {
     return 'set-config';
   }
 
   public function getCommandSynopses() {
     return phutil_console_format(<<<EOTEXT
       **set-config** [__options__] -- __name__ __value__
 EOTEXT
       );
   }
 
   public function getCommandHelp() {
     return phutil_console_format(<<<EOTEXT
           Supports: cli
           Sets an arc configuration option.
 
           Options are either user (apply to all arc commands you invoke
           from the current user) or local (apply only to the current working
           copy). By default, user configuration is written. Use __--local__
           to write local configuration.
 
           User values are written to '~/.arcrc' on Linux and Mac OS X, and an
           undisclosed location on Windows. Local values are written to an arc
           directory under either .git, .hg, or .svn as appropriate.
 EOTEXT
       );
   }
 
   public function getArguments() {
     return array(
       'local' => array(
         'help' => pht('Set a local config value instead of a user one.'),
       ),
       '*' => 'argv',
     );
   }
 
   public function requiresRepositoryAPI() {
     return $this->getArgument('local');
   }
 
   public function run() {
     $argv = $this->getArgument('argv');
     if (count($argv) != 2) {
       throw new ArcanistUsageException(
         pht('Specify a key and a value.'));
     }
     $configuration_manager = $this->getConfigurationManager();
 
     $is_local = $this->getArgument('local');
 
     if ($is_local) {
       $config = $configuration_manager->readLocalArcConfig();
       $which = 'local';
     } else {
       $config = $configuration_manager->readUserArcConfig();
       $which = 'user';
     }
 
     $key = $argv[0];
     $val = $argv[1];
 
     $settings = new ArcanistSettings();
 
+    $console = PhutilConsole::getConsole();
+
+    if (!$settings->getHelp($key)) {
+      $warn = pht(
+        'The configuration key \'%s\' is not recognized by arc. It may '.
+        'be misspelled or out of date.',
+        $key);
+      $console->writeErr("**%s:** %s\n", pht('Warning'), $warn);
+    }
+
     $old = null;
     if (array_key_exists($key, $config)) {
       $old = $config[$key];
     }
 
     if (!strlen($val)) {
       unset($config[$key]);
       if ($is_local) {
         $configuration_manager->writeLocalArcConfig($config);
       } else {
         $configuration_manager->writeUserArcConfig($config);
       }
 
       $old = $settings->formatConfigValueForDisplay($key, $old);
 
       if ($old === null) {
-        echo pht(
+        $console->writeOut(
           "Deleted key '%s' from %s config.\n",
           $key,
           $which);
       } else {
-        echo pht(
+        $console->writeOut(
           "Deleted key '%s' from %s config (was %s).\n",
           $key,
           $which,
           $old);
       }
     } else {
       $val = $settings->willWriteValue($key, $val);
 
       $config[$key] = $val;
       if ($is_local) {
         $configuration_manager->writeLocalArcConfig($config);
       } else {
         $configuration_manager->writeUserArcConfig($config);
       }
 
       $val = $settings->formatConfigValueForDisplay($key, $val);
       $old = $settings->formatConfigValueForDisplay($key, $old);
 
       if ($old === null) {
-        echo pht(
+        $console->writeOut(
           "Set key '%s' = %s in %s config.\n",
           $key,
           $val,
           $which);
       } else {
-        echo pht(
+        $console->writeOut(
           "Set key '%s' = %s in %s config (was %s).\n",
           $key,
           $val,
           $which,
           $old);
       }
     }
 
     return 0;
   }
 
 }