diff --git a/src/applications/config/check/PhabricatorBaseURISetupCheck.php b/src/applications/config/check/PhabricatorBaseURISetupCheck.php
--- a/src/applications/config/check/PhabricatorBaseURISetupCheck.php
+++ b/src/applications/config/check/PhabricatorBaseURISetupCheck.php
@@ -9,20 +9,53 @@
   protected function executeChecks() {
     $base_uri = PhabricatorEnv::getEnvConfig('phabricator.base-uri');
 
-    if (strpos(AphrontRequest::getHTTPHeader('Host'), '.') === false) {
-      $summary = pht(
-        'The domain does not contain a dot. This is necessary for some web '.
-        'browsers to be able to set cookies.');
-
-      $message = pht(
-        'The domain in the base URI must contain a dot ("."), e.g. '.
-        '"http://example.com", not just a bare name like "http://example/". '.
-        'Some web browsers will not set cookies on domains with no TLD.');
+    $host_header = AphrontRequest::getHTTPHeader('Host');
+    if (strpos($host_header, '.') === false) {
+      if (!strlen(trim($host_header))) {
+        $name = pht('No "Host" Header');
+        $summary = pht('No "Host" header present in request.');
+        $message = pht(
+          'This request did not include a "Host" header. This may mean that '.
+          'your webserver (like nginx or apache) is misconfigured so the '.
+          '"Host" header is not making it to Phabricator, or that you are '.
+          'making a raw request without a "Host" header using a tool or '.
+          'library.'.
+          "\n\n".
+          'If you are using a web browser, check your webserver '.
+          'configuration. If you are using a tool or library, check how the '.
+          'request is being constructed.'.
+          "\n\n".
+          'It is also possible (but very unlikely) that some other network '.
+          'device (like a load balancer) is stripping the header.'.
+          "\n\n".
+          'Requests must include a valid "Host" header.');
+      } else {
+        $name = pht('Bad "Host" Header');
+        $summary = pht('Request has bad "Host" header.');
+        $message = pht(
+          'This request included an invalid "Host" header, with value "%s". '.
+          'Host headers must contain a dot ("."), like "example.com". This '.
+          'is required for some browsers to be able to set cookies.'.
+          "\n\n".
+          'This may mean the base URI is configured incorrectly. You must '.
+          'serve Phabricator from a base URI with a dot (like '.
+          '"https://phabricator.mycompany.com"), not a bare domain '.
+          '(like "https://phabricator/"). If you are trying to use a bare '.
+          'domain, change your configuration to use a full domain with a dot '.
+          'in it instead.'.
+          "\n\n".
+          'This might also mean that your webserver (or some other network '.
+          'device, like a load balancer) is mangling the "Host" header, or '.
+          'you are using a tool or library to issue a request manually and '.
+          'setting the wrong "Host" header.'.
+          "\n\n".
+          'Requests must include a valid "Host" header.',
+          $host_header);
+      }
 
       $this
-        ->newIssue('config.phabricator.domain')
-        ->setShortName(pht('Dotless Domain'))
-        ->setName(pht('No Dot Character in Domain'))
+        ->newIssue('request.host')
+        ->setName($name)
         ->setSummary($summary)
         ->setMessage($message)
         ->setIsFatal(true);