diff --git a/src/applications/project/__tests__/PhabricatorProjectCoreTestCase.php b/src/applications/project/__tests__/PhabricatorProjectCoreTestCase.php
index f6b74801e6..8c17fc07e7 100644
--- a/src/applications/project/__tests__/PhabricatorProjectCoreTestCase.php
+++ b/src/applications/project/__tests__/PhabricatorProjectCoreTestCase.php
@@ -310,7 +310,6 @@ final class PhabricatorProjectCoreTestCase extends PhabricatorTestCase {
     $slugs = mpull($slugs, 'getSlug');
 
     $this->assertTrue(in_array($name2, $slugs));
-
   }
 
   public function testDuplicateSlugs() {
diff --git a/src/applications/project/editor/PhabricatorProjectTransactionEditor.php b/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
index ff9b23e477..70daac64ed 100644
--- a/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
+++ b/src/applications/project/editor/PhabricatorProjectTransactionEditor.php
@@ -268,6 +268,17 @@ final class PhabricatorProjectTransactionEditor
         }
 
         $name = last($xactions)->getNewValue();
+
+        if (!PhabricatorSlug::isValidProjectSlug($name)) {
+          $errors[] = new PhabricatorApplicationTransactionValidationError(
+            $type,
+            pht('Invalid'),
+            pht(
+              'Project names must contain at least one letter or number.'),
+            last($xactions));
+          break;
+        }
+
         $name_used_already = id(new PhabricatorProjectQuery())
           ->setViewer($this->getActor())
           ->withNames(array($name))
@@ -304,6 +315,27 @@ final class PhabricatorProjectTransactionEditor
         $slug_xaction = last($xactions);
 
         $new = $slug_xaction->getNewValue();
+
+        $invalid = array();
+        foreach ($new as $slug) {
+          if (!PhabricatorSlug::isValidProjectSlug($slug)) {
+            $invalid[] = $slug;
+          }
+        }
+
+        if ($invalid) {
+          $errors[] = new PhabricatorApplicationTransactionValidationError(
+            $type,
+            pht('Invalid'),
+            pht(
+              'Hashtags must contain at least one letter or number. %s '.
+              'project hashtag(s) are invalid: %s.',
+              phutil_count($invalid),
+              implode(', ', $invalid)),
+            $slug_xaction);
+          break;
+        }
+
         $new = $this->normalizeSlugs($new);
 
         if ($new) {
diff --git a/src/infrastructure/internationalization/translation/PhabricatorUSEnglishTranslation.php b/src/infrastructure/internationalization/translation/PhabricatorUSEnglishTranslation.php
index 762910413c..ff4174522c 100644
--- a/src/infrastructure/internationalization/translation/PhabricatorUSEnglishTranslation.php
+++ b/src/infrastructure/internationalization/translation/PhabricatorUSEnglishTranslation.php
@@ -774,6 +774,14 @@ final class PhabricatorUSEnglishTranslation
       '%s changed project hashtag(s), added %d: %s; removed %d: %s.' =>
         '%s changed project hashtags, added %3$s; removed %5$s.',
 
+      'Hashtags must contain at least one letter or number. %s '.
+      'project hashtag(s) are invalid: %s.' => array(
+        'Hashtags must contain at least one letter or number. The '.
+        'hashtag "%2$s" is not valid.',
+        'Hashtags must contain at least one letter or number. These '.
+        'hashtags are invalid: %2$s.',
+      ),
+
       '%s added %d project hashtag(s): %s.' => array(
         array(
           '%s added a hashtag: %3$s.',
diff --git a/src/infrastructure/util/PhabricatorSlug.php b/src/infrastructure/util/PhabricatorSlug.php
index fd169914fe..c977c21d70 100644
--- a/src/infrastructure/util/PhabricatorSlug.php
+++ b/src/infrastructure/util/PhabricatorSlug.php
@@ -8,6 +8,11 @@ final class PhabricatorSlug extends Phobject {
     return rtrim($slug, '/');
   }
 
+  public static function isValidProjectSlug($slug) {
+    $slug = self::normalizeProjectSlug($slug);
+    return ($slug != '_');
+  }
+
   public static function normalize($slug, $hashtag = false) {
     $slug = preg_replace('@/+@', '/', $slug);
     $slug = trim($slug, '/');