diff --git a/src/applications/differential/query/DifferentialRevisionQuery.php b/src/applications/differential/query/DifferentialRevisionQuery.php
--- a/src/applications/differential/query/DifferentialRevisionQuery.php
+++ b/src/applications/differential/query/DifferentialRevisionQuery.php
@@ -466,13 +466,25 @@
       $basic_authors = $this->authors;
       $basic_reviewers = $this->reviewers;
 
+      // Note that '$this->responsibles' can contain both projects and users,
+      // so let's get only the users.
+      $responsible_users = $this->filterUserPHIDs($this->responsibles);
+
       try {
+        // First part of the UNION query:
         // Build the query where the responsible users are authors.
-        $this->authors = array_merge($basic_authors, $this->responsibles);
-
+        // Special case: skip this first query if it does not have any
+        // condition, because it would just return everything,
+        // and we would generate "(SELECT EVERYTHING) UNION (SELECT SOMETHING)"
+        // and this first nonsense query returning everything would defuse the
+        // next query returning something.
+        $this->authors = array_merge($basic_authors, $responsible_users);
         $this->reviewers = $basic_reviewers;
-        $selects[] = $this->buildSelectStatement($conn);
+        if (!$this->authors && !$this->reviewers) {
+          $selects[] = $this->buildSelectStatement($conn);
+        }
 
+        // Second part of the UNION query:
         // Build the query where the responsible users are reviewers, or
         // projects they are members of are reviewers.
         $this->authors = $basic_authors;
@@ -1080,6 +1092,16 @@
     return $user_authority + $project_authority + $package_authority;
   }
 
+  /**
+   * Get a list of PHIDs, and return only user PHIDs.
+   * @param array $phids Mixed PHIDs
+   * @return array PHIDs User PHIDs
+   */
+  private function filterUserPHIDs(array $phids): array {
+    $phids_grouped = phid_group_by_type($phids);
+    return idx($phids_grouped, PhabricatorPeopleUserPHIDType::TYPECONST, []);
+  }
+
   public function getQueryApplicationClass() {
     return PhabricatorDifferentialApplication::class;
   }